iproto: implement interactive transactions over iproto streams

Implement interactive transactions over iproto streams. Each stream
can start its own transaction, so they allows multiplexing several
transactions over one connection. If any request fails during the
transaction, it will not affect the other requests in the transaction.
If disconnect occurs when there is some active transaction in stream,
this transaction will be rollbacked, if it does not have time to commit
before this moment.

Part of #5860

@TarantoolBot document
Title: interactive transactions was implemented over iproto streams.
The main purpose of streams is transactions via iproto. Each stream
can start its own transaction, so they allows multiplexing several
transactions over one connection. There are multiple ways to begin,
commit and rollback transaction: using IPROTO_CALL and IPROTO_EVAL
with corresponding function (box.begin, box.commit and box.rollback),
IPROTO_EXECUTE with corresponding sql request ('TRANSACTION START',
'COMMIT', 'ROLLBACK') and IPROTO_BEGIN, IPROTO_COMMIT, IPROTO_ROLLBACK
accordingly. If disconnect occurs when there is some active transaction
in stream, this transaction will be rollbacked, if it does not have time
to commit before this moment. Add new command codes for begin, commit and
rollback transactions: `IPROTO_BEGIN 14`, `IPROTO_COMMIT 15` and
`IPROTO_ROLLBACK 16` accordingly.