relay: synchronize raft and promote term dispatch

When processing promote/demote row from wal in relay, process tx
endpoint until last sent raft term is greater or equal to term in
promote/demote request.

Part of #6754

@TarantoolBot document
Title: box.info.synchro.queue.term added

Added `term` field to `box.info.synchro.queue`. It contains term of the
last PROMOTE. It is usually equal to `box.info.election.term` but may be
less than election term when new round of elections started, but no one
promoted yet.