Review fixes.
Part of #2435.

* better messages
* fold sql_prepare()
* rewrite the message decode

Add iproto_execute command to get not only sql rows as the result,
but also sql meta data about each result column.

Make SQLITE_LIMIT_VARIABLE_NUMBER be static

SQLITE_LIMIT_VARIABLE_NUMBER is the run time limit of the
variable count in the query. The Tarantool doesn't support dynamic
limits for such things. Let's make this limit be enum instead
of value in the limits array (struct sqlite3.aLimit).

Part of #2285.

Add `sqlite3BitvecBuiltinTest` to exported symbols to expose
it to ffi. Bug #2590 submitted to remove it from exports.

Raise syntax error in a case of empty requests. Update sql-tokenizer
to support exising tests.

Test alias.test.lua is fully disabled, see #2582

Part of #2381

Instead introduce vy_run_env_enable_coio(), which starts reader threads,
and make vy_run_iterator automatically switch to coio if reader threads
are running. With this patch, vy_read_iterator doesn't need a pointer to
vy_env to add a run as a source, only vy_run_env.

While we are at it, cleanup vy_conf a bit.

Needed for #1906

vy_run_write_page() doesn't take a reference to last_stmt, because it
assumes that the write iterator guarantees it won't be deleted until
'next' is called again. The iterator does pin a statement if it is read
from a run file - see vy_write_iterator_set_tuple() - however there's a
case when the last returned statement can go away under us. This will
happen if the iterator is used for major compaction and the last source
statement is a DELETE. In this case the iterator will unreference the
last statement it returned to the caller, take a reference to the DELETE
instead, but won't return the DELETE - see vy_write_iterator_next(). As
a result, the caller, i.e. vy_run_write_page(), will hit use-after-free
on an attempt to read last_stmt.

To fix this bug, let's make vy_run_write_page() take a reference to
last_stmt as it used to before the write iterator was reworked. A test
case will be added later, after all iterator-related issues have been
fixed.

Closes #2578

Iconv is a library to convert a sequence of characters in one
character encoding to a sequence of characters in another character
encoding. Example below converts utf-16 big endian string into utf-8
string:

    convertor = require('iconv').new('UTF-16BE', 'UTF-8')
    converted_string = convertor(source_string)

Closes #2587

vy_task_dump_new() deletes empty in-memory trees right away, but
doesn't increment vy_index->mem_list_version, which may result in
a read iterator crash accessing a deleted vy_mem.

Now open MP sort is used for any size of an array.
For small arrays it's an overkill and even can cause overhead
due to thread pool creation.

Invoke open MP sort only for big arrays and use old good
single-thread qsort for small arrays.

Fix #2431

Print original uri as is if it doesn't contain sensitive
information.

Closes #2292

Currently, quota (and some global stats) are adjusted in
vy_index_commit_upsert(), vy_tx_prepare(), and vy_tx_commit(),
which entails dependency of vy_index and tx_manager on vy_env.
Let's move this code to the upper level, i.e. to vy_prepare()
and vy_commit().

Needed for #1906

Remove box.info.vinyl().performance.{tx_rlb,tx_conflict}. Instead add
'tx' section to box.info.vinyl() with the following counters:

 - 'active': number of active transactions
 - 'commit': number of committed transactions
 - 'rollback': number of rolled back transactions
 - 'conflict': number of transactions aborted on conflict

Apart from improving vinyl statistics, this patch also pursues one more
goal: removing dependency of vy_tx and tx_manager on vy_env, which is
needed to move them to a separate source file.

Needed for #1662 #1906

It belongs there, really.

Currently, during local recovery, an index is added to the
scheduler from vy_index_recover(), which makes this function
depend on index->env->scheduler. Let's postpone addition to
vy_index_commit_create() as we do when vinyl is online.

Needed for #1906

Needed to remove dependency of vy_index on struct txv.

Needed for #1906

Before this patch, the new space, created by alter specification,
would be put into space cache only after successful WAL write.

This behaviour is not linearizable: on a replica, the WAL is
played sequentially, and the order of events could differ from the
master.

Besides, it could crash, as demonstrated in gh-2074 test case.

Since we use a cascading rollback for all transactions on WAL
write error, it's OK to put a space into space cache
before WAL write, so that the new transactions apply to the new
space.

This patch does exactly that.

All subsequent requests are executed against the new space.

This patch also removes on_replace trigger in the old space, since
all checks against the new tuple format are performed using the new
space.

Fixes #2074.

* UTF-16 support removed
* Tests with UTF-16* cases removed

Closes #2295

 Do not leak input buffer references on an incorrect packet.

 Before this patch, in case of incorrect packet, in->rpos was never
 advanced, creating an eternal request and blocked input buffer.

 A follow up on gh-1654.

closes gh-2576

* update comments
* add a test case for altering a primary key on the fly
* rename AddIndex to CreateIndex
* factor out common code into a function

MoveIndex operation is used to move an existing index from the old space
to the new one. Semantically it's a no-op.

RebuildIndex is introduced for case when essential index properties are
changed, so it is necessary to drop the old index and create a new one in
its place in the new space.

AlterSpaceOp::prepare() is removed: all checks are moved to
on_replace trigger in _index system space from it.
All checks are done before any alter operation is created.

Necessary for gh-2074 and gh-1796.

Move the check which decides on an alter strategy whenever
a row in _index space is changed, from AlterSpaceOp::preapre()
to on_replace trigger on _index space.

The check chooses between two options: a heavy-weight index rebuild,
invoked when index definition, such as key parts, is changed, vs.
lightweight modify, invoked when index name or minor options are
modified..

Before this patch, index alteration creates a pair of operations
(DropIndex + AddIndex) in all cases, but later
replaces two operations with one at AlterSpaceOp::prepare() phase.

This is bad by several reasons:

- it's done while traversing of a linked list of operations,
  and it changes the list being traversed.

- an order in the list of operations is required for this
to work: drop must precede add.

- needless allocation and deallocation of operations makes the logic
  unnecessarily complex.

Necessary for gh-1796.

Always first create the primary key index in a space. Put
the primary key key def first in the array of key_defs, passed
into tuple_format_new(). This is necessary for gh-1796.

Assert that we can't create a space with secondary key but no primary.

Now drop primary index checks are made in alter triggers after
new space creation. Such an implementation leads to temporary
creation of a space with invalid index set. Fix it and check
the index set before space_new call.

Non destructive swap_index_def function sould be used because memtx_tree
stores a pointer to an index_def used while tree creation. Fixed #2570

Use `net.box` instead of `console`.

LLVM is more eager for frame size than GCC
Current maximal call chain length may corrupt stack then.
Reduce maximal call chain length to 30.

Related to #2550

If after recovery first client to invoke SQL differs from
ADMIN, then initialization of SQL subsystem will fail due to
insufficient access rights. Problem is that SQL invokes
create_from_tuple() to fill space_def structure, which in turn
verifies access right.
As far as SQL is a static part of Tarantool, there's no sense
to initialize the system by demand. Change performs data dictionary
initialization upon config completion.
Regression test added as well.

Closes #2483