sql: Support per-statement transactions

Introduce dedicated opcode OP_TTRansaction, which is
responsible for initiation of Tarantool transaction.
This opcode is only emitted if statement is generated
directly by parser. That means that changes like insertions
into system tables are not covered by Tarantool's transations.

Tarantool's transaction is committed or rolled back during
exectuin of `Halt` opcode. `CloseAllCursors` function was
splitted into two: in order Tarantool's WAL to work need to
preserve allocated memory until commit to WAL. Call this
deallocation after Tarantool's commit.
In future this will be re-worked since transaction can be
either in statement or generic, and not all Halts should commit.

Also, fix a bug: memory area passed to WAL were re-used. WAL itself
doesn't do memcpy, so this state of affairs was lead to unpredictable
errors during save/restore. Use region_alloc to reserve memory area
for WAL in tarantoolSqlite3Insert and tarantoolSqlite3Delete.

Closes #2599