Before this patch, resulting type of concatenation operator always was
TEXT (i.e. type of memory cell containing result - MEM_Str). Lets fix it
and return type depending on type of concatenation arguments: if both
arguments are TEXT, then resulting type is TEXT; BLOB otherwise. Note
that other options of combining types of arguments are illegal.

Closes #3544

Original SQLite operator of concatenation accepts all types of
arguments. If type of parameter is not TEXT, it is implicitly converted
to TEXT (except for NULLs). That contradicts ANSI (it is regulated by
[1]), so lets allow only TEXT and BLOB as argument type for
concatenation.  Moreover, they both must be of the same type at the same
time (i.e. both TEXT or BLOB).

[1] SQL ANSI 2013, 9.5 Result of data type combination

Part of #3544

Accidentally, mask which is used to map type of VDBE memory cell into
outer API types was replaced with MEM_TypeMask. But value of the latter
is larger then possible values of VDBE memory cells types. Hence, if it
is applied to memory cell, not only pure types is taken into
consideration, but some additional flags (such as MEM_Zero) as well.
Overall, it results in wrong type calculation for zeroed blobs, for
instance. Lets return back previous mask.

Follow-up #3698
Needed for #3544

yaml.encode() now wraps a string literal whose content is equal to a
null or a boolean value representation in YAML into single quotes. Those
literals are: 'false', 'no', 'true', 'yes', '~', 'null'.

Reverted the a2d7643c commit to use single quotes instead of multiline
encoding for 'true' and 'false' string literals.

Follows up #3476
Closes #3662
Closes #3583

Use lua_is*() functions instead of explicit lua_gettop() checks in
yaml.encode() and yaml.decode() functions.

Behaviour changes:

* yaml.decode(object, nil) ignores nil (it is consistent with encode
  behaviour).
* yaml.encode() gives an usage error instead of "unsupported Lua type
  'thread'".
* yaml.encode('', {}, {}) ignores 3rd argument (it is consistent with
  decode behaviour).

Previously readahead was set after the instance synced with all existing
masters. This could lead to a case when some client connections created
rather early would start with default readahead value even when
it was explicitly set to some value.
So, start setting readahead before replicaset sync.

Follow-up: #3958

Iproto connections keep old readahead values for input buffers even
after box.cfg.readahead reconfiguration. This means that for the changes
to take place for the old clients, they have to reconnect. Otherwise
tarantool log will be spammed with 'readahead limit is reached' errors.

To fix this, start updating input buffer size for iproto connections
if needed every time the buffer is empty.

Closes: #3958

Checks for fiber_name legth worked with FIBER_NAME_MAX,
while one byte in fact was occupied by terminating zero.

Closes #4011

vy_run_iterator_seek left the return statement uninitialized under
certain conditions. The branch in the code wasn't covered by any test,
because it could only be triggered by an EQ SELECT following range
coalescing, so we only saw it when a customer reported a crash.
Fix it and improve vinyl/select_consistency test to cover this branch.

Reserve() function by definition should ensure that there is
enough space to store something. But heap reserve() always just
increases the capacity twice. Even if there was already enough
memory. Check for not necessary realloc was done out of this
function. This was wrong. The commit makes reserve() be real
reserve().

The commit is motivated not by pursue of justice, but by
forthcoming usage of reserve() in SWIM module code.

Now heap API works with struct heap_node only, which forces a
user to constantly call container_of. Such a code looks really
awful. This commit makes heap taking and returning user defined
structures, and removes container_of clue.

It is worth noting, that the similar API rb-tree and b-tree
have. Even rlist has its rlist_*_entry() wrappers, and mhash
provides macroses to define your own value type.

During DDL routines we pass encoded space/index/trigger formats
into msgpack to VDBE. EXPLAIN query displays arguments of each opcode of
VDBE program in a readable format. So, lets decode arguments of OP_Blob
opcode with subtype = _MSGPACK before displaying them. Also, lets
enlarge static buffers for P4 operand value and opcode comment to fit
decoded msgpack.

What is more, it fixes buffer-overflow since before this patch operands
of OP_Blob were treated as strings and passed to functions like strlen()
(only during EXPLAIN query). On the other hand, generally speaking
msgpack can come without null termination, or contain '\0' symbols in
the middle of encoded array.

Closes #3868

According to ANSI, result of concatenation operation should derive
collation sequence from its operands. Now it is not true: result is
always comes with no ("none") collation.

In a nutshell[1], rules are quite simple:

a) If some data type has an explicit collation EC1, then every data type
that has an explicit collation shall have declared type collation that
is EC1.  The collation derivation is explicit and the collation is EC1.

b) If every data type has an implicit collation, then:

 - If every data type has the same declared type collation IC1, then
   the collation derivation is implicit and the collation is IC1.

 - Otherwise, the collation derivation is none.

c) Otherwise, the collation derivation is none.

[1] Read complete statement at chapter 9.5 Result of data type
combinations, ANSI 2013, Part 2: Foundations.

Closes #3937

Lets make sql_expr_coll() return error code. It is needed since we are
going to use this function to detect operands of concatenation with
incompatible collations. Hence, struct coll* has been moved to output
parameters.  For the same reason lets make sql_binary_compare_coll_seq()
return error code instead of struct coll*. The latter is really required
only in one out of six usages, so lets remove it at all.

Needed for #3947

Part of #3271

Rename struct fkey and struct fkey_def to fk_constraint and
fk_constraint_def respectively.
Consistently use "fk" for foreign key variables, and fk_def for
foreign key definition variables.

Remove dependency of fkey.h on space.h.
Enfore subject-verb-object naming in a few fk-related methods.

Follow up on the patch adding transaction boundaries to xrow stream.
Use tsn as an abbreviation for transaction identifier (transaction
sequence number). It is an important enough concept to use a short and
a convenient tag name for.
Deploy the name across the code - in names and comments.
Clarify comments.
Still use box_txn_id() as API method since box_tsn() and box_txn() would
be too easy to mistype.

SQLite grammar implementing foreign keys parsing is quite compact, but
on the other hand it allows to define ON DELETE and ON UPDATE actions
multiple times. For instance:

... REFERENCES t ON DELETE UPDATE ON DELETE RESTRICT;

It makes no sense and contradicts original ANSI syntax. So, lets rework
it a bit. Firstly, MATCH clause must come first, so we place it in
independent rule. Then we remove ON INSERT clause, since there is no
such opportunity at all. Finally, we have only 4 options to expose
refargs (i.e. grammar rule to parse FK actions): ON UPDATE, ON DELETE,
ON UPDATE ON DELETE, ON DELETE ON UPDATE. That's it.

Closes #3475

Append txn_id and is_commit to xrow_header structure, txn_id identifies
transaction id on replica where transaction was started. As transaction id
a lsn of the first row in the transaction is used. is_commit is set to true
for the last row in a transaction.

As encoding/deconding rule assumed:
 * txn_id encoded using transaction sequence number iproto field
   as IPROTO_TSN = lsn - txn_id,
 * is_commit packed into IPROTO_FLAGS field with a bit mask,
 * txn_id and is_commit are encoded only for multi-row transactions.
   So if we do not have txn_id after row decoding then this means that it
   is a single row transaction.

These rules provide compatibility with previous xlog format as well
as good compaction level.

Needed for #2798

The function doesn't require any knowledge of vinyl statement layout and
can work on regular tuples. Let's rename it to tuple_key_contains_null,
move its implementation to tuple_extract_key.cc, and declare it in
key_def.h, as we do with other similar functions.

Most of them are already there - for instance see see tuple_extract_key
and tuple_compare. Let's move the rest there too for consistency.

 - Rename key_def_set_cmp to key_def_set_func, because it sets not only
   comparators these days.
 - Rename tuple_hash_func_set and tuple_extract_key_set to
   key_def_set_hash_func and key_def_set_extract_func, because it's more
   like subject-verb-object naming convention used throughout the code.
 - Introduce key_def_set_compare_func and use it instead of setting
   comparators explicitly in key_def.c.

It isn't used anywhere outside vy_stmt.c.

No need to allocate an empty key during DDL - we already have one
preallocated in vy_lsm_env.

Add optional 'chars' parameter to string.strip, string.lstrip
and string.rstrip for specifying the unwanted characters.
Behavior modeled after the equivalent Python built-ins.

Closes: #2977

@TarantoolBot document
Title: string.strip(inp, chars)
Update the documentation for string.strip,
string.lstrip and string.rstrip to reflect
the addition of the optional param.

This patch adds space name to descriptions of some of new errors.
Also it fixes name and description of a few errors.

Part of #3965

When a connection is closed, it should not allow any
requests - async and not. But before this patch this error
from netbox.perform_async_request was ignored.

This patch reworks SQL errors of types "no such object" and
"object exists". After this patch, these error will be set as
Tarantool errors.

Part of #3965

The suppressErr field was used to indicate that most of the errors
during the parsing should be suppressed. There was only one
feature that used it. After deleting this feature, it became
obvious that this field had become unused and should be removed.

The feature in question is: allow to use names and aliases from
input in ORDER BY clause of UNION or INTERSECT. After deleting
of this feature, requests like the one below become invalid:

SELECT 1 AS a UNION ALL SELECT 2 AS b ORDER BY b;

Part of #3965

The pragmas "query_only" and "read_uncommitted" didn't affect anything
and were removed.
Fixed an error in pragma index_list which caused a segmantation fault.
pragma sql_default_engine accepts only strings.
Thus pragma sql_default_engine('memtx') is a well-formed command,
while pragma sql_default_engine(memtx) or
pragma sql_default_engine("memtx") are considered as an ill-formed and
raise an error.

Closes #3733

The box.tuple.new() used to call luamp_encode_tuple with
default LUA serializer config 'luaL_msgpack_default'. This
routine may consider an array to be excessively sparse when
  + encode_sparse_ratio > 0
  + max(table) > encode_sparse_safe
  + max(table) > count(table) * encode_sparse_ratio.
Sparse optimization save memory via representing excessively
sparse tuple as MP_MAP. But Tarantool tuple always must be
MP_ARRAY so it is not relevant for box.tuple.new semantics.
So it is disabled with encode_sparse_ratio = 0 in a new local
serializer config.

Closes #3882

It is going to be used in SWIM module to set UDP server socket
options. Apparently this function sets some usefull flags like
NONBLOCK, REUSEADDR.

The function parses string URI consisting of either IP and port,
or UNIX socket address, and stores the result into struct
sockaddr.

Set the reason "Unknown" when it is CURLE_OK and status is more than or equal
to 400.

Closes #3681

A bug existed because module_init was called during a call to box_cfg{}.
Modules were not initialized before calling box.cfg{}.

Closes #3770

Lets completely remove struct Table. Also the patch simplifies memory
management as in many cases struct space (which replaces struct Table)
is allocated on region and shouldn't be explicitly freed.  Some wrappers
fetching data from space (such as space_checks_expr_list) have been
removed since now we can get all properties right from space object
right from cache.

Closes #3235

This patch makes logging a little bit more verbose so as to facilitate
post-mortem replication failure analysis:

 - When an instance starts up, print its uuid and vclock.
 - When a replica connects to a master, print the master's uuid to
   the replica's log.
 - Print replica uuid and address to the master's log on join/subscribe.
 - Log local and remote vclock on subscribe, both on master and on
   replica.
 - Log ids assigned to replicas both on recovery and when online.
   Also, log when a replica is deleted from the cluster.