Transaction support for applier

Applier fetch incoming rows to form a transaction and then apply it.
Rows are fetched and stored on fiber gc region until last transaction row
with is_commit was fetched. After fetch a multi row transaction is going to be
applied into txn_begin/txn_commit/txn_rolback boundaries. At this time
we could not apply single row transaction in such boundaries because of
ddl which does not support non auto commit transactions.

Closes: #2798
Needed for: #980