Preparation for converting txn to C.

This will help removing exceptions from txn methods.

Needed for #2776

To convert engine and space infrastructure to C, we need to make txn
usable from C code. The only reason why we can't convert txn.cc to C
right now is triggers: they are set from alter.cc and may throw
exceptions. To handle this, let's make trigger_run() catch all
exceptions and return an error code instead. For C++ code, introduce
trigger_run_xc() which calls trigger_run() under the hood and throws
the exception suppressed by it.

Needed for #2776

Preparation for converting engine implementation to C.

Needed for #2776

 - Add a return code to both space_foreach() and its callback so that
   the caller can use it to propagate errors instead of throwing
   exceptions.
 - Make space_foreach() exception-free.
 - Export the function to C.

Preparation for converting engine implementation to C.

Needed for #2776

Preparation for converting engine implementation to C.

This will help removing exceptions from engine callbacks.

Needed for #2776

vy_set_timeout() and vy_set_max_tuple_size() are not supposed to fail.
Remove the return code and diag_raise() in the engine wrapper.

Needed for #2776

Preparation for converting class Engine to plain struct.

Needed for #2776

engine::createFormat() wasn't such a good idea after all - it's
difficult to convert it to C in a neat way: the problem is it may
return NULL on success (which it does in case of sysview engine)
so to make it exception-free we have to either return the format
in an extra argument or allow the engine set this method to NULL.
Both ways look ugly.

Let's remove engine::createFormat() and oblige engine::createSpace()
fully initialize a space, similarly to how it works for indexes. To
ease its work, introduce space_create() which initializes engine
independent fields (i.e. struct space).

While we are at it, let's also
 - make memtx/vinyl space_vtab private to the engines and export
   functions to create a memtx/vinyl space;
 - make struct space initialization code exception-free - after all
   we are going to convert it to C.

Needed for #2776

Vinyl engine initialization is done in two steps: first we create the
engine class and then we call ->init() method, which creates vinyl
environment. There's no reason not to create the environment right in
the engine class constructor, so let's move it there and drop ->init().
Also, let's pass engine parameters to the engine constructor, just like
we do in case of memtx, instead of reading the config from the engine
implementation.

To find an engine object, we always use engine_find(), which raises an
error if the engine was not found. Sometimes we know for sure that the
engine we are looking for exists, in which case we don't need to raise
an error. Let's introduce engine_by_name() that just looks up the engine
and does nothing else and use it when we are sure that the engine must
exist.

Rename MemtxEngine, VinylEngine, and SysviewEngine to memtx_engine,
vinyl_engine, and sysview_engine as well.

Preparation for converting class Engine to plain struct.

Needed for #2776

IllegalParams and ErrorInjection are not used anymore. Delete them.

Preparation for converting space implementation to C.

Needed for #2776

There are four such functions declared in space.h:

  space_new
  index_find_unique
  index_find_system
  access_check_space

index_find_unique() and access_check_space() are used in the code that
is about to be converted to C so for them add exception-free variants
(without xc suffix).

Preparation for converting space implementation to C.

Needed for #2776

This function is very unreliable:
 - it crashes if the space doesn't have indexes;
 - it only works for memtx spaces, for other engines
   it throws an exception.

Since it is only used in memtx, let's zap it and call index_size()
directly.

Preparation for converting space implementation to C.

Note, all methods that may throw have *_xc suffix.
This will help removing exceptions from space callbacks.

Needed for #2776

Needed for #2776

Add index_build_xc() wrapper for C++ code.

Preparation for converting index implementation to C.

Needed for #2776

Preparation for converting index implementation to C.

Needed for #2776

Add space_cache_find_xc() wrapper for C++ code.

Preparation for converting index implementation to C.

Needed for #2776

Needed for #2776

Preparation for converting class Index to plain struct.

Note, all methods that may throw have *_xc suffix.
This will help removing exceptions from index callbacks.

Needed for #2776

 - Index::findByTuple(), index_is_primary(), index_id() are not used
   anywhere.
 - Index::initIterator() is useless - it just throws an exception, child
   classes can do it themselves.
 - index_name() is trivial - callers can use index->def->name.

Needed for #2776

For the sake of aesthetics and consistency with space::def.

Needed for #2776

Preparation for converting class Index to plain struct.

Needed for #2776

Preparation for converting class Index to plain struct.

Needed for #2776

Preparation for converting class Index to plain struct.

Needed for #2776

Closes #2832

Closes #2773

After implementing of #2802 many user started to complain about
sudden change of _index system space format.

Since both formats (new and old) are fully supported it is possible
to store part definition in old format if there are no collation or
other special options. Make it.

Closes #2802

- Forbid get(), update(), delete() with NULLS
- Improve is_nullable checking in ALTER
- Set default value for is_nullable from space:format()
- Rename primary_key_validate() to exact_key_validate()

Follow up #1557

Use extended key parts to compare tuples, containing NULLs in the
base key parts.

Closes #1557

Unique part count is a minimal part count which always is unique.
For example, if a secondary index is unique, then
unique_part_count == secondary index part count. But if a secondary
index is not unique, then unique_part_count == part count of a merged key_def.

Needed for #1557

Store nullable in field_def, tuple_field, key_def and key_part.
Key_def is nullable if at least one part references to a nullable field.

Needed for #1557

If the only change is a new type of a key_def part and a new type
can store values of an old type, then do not rebuild index.

Disallow changing field type of space format fields to an
incompatible type.
For example, forbid changing a type from integer to string,
but allow to change from integer to number.

If an user adds new fields, then validate existing tuples to be
compatible with a new format. Check works in O(N) time and for
big spaces can be realy long. But it can be skipped, if conditions
below are ok:
1. if space format field count < old tuple format field count. Then
   field count of all tuples >= new format field count;
2. new fields are indexed or have type ANY. If a field is already
   indexed, then its type is already validated. If a field has
   type ANY, then any value is ok.

Closes #2800

Fast ucol_strcollUTF8 function was introduced in ICU 50. In older
ICU versions we have to use slower ucol_strcollIter with properly
initialized UTF8 iterators.

Fix #2820

Follow up the previous commit