replication: persist confirmed vclock on replicas

Previously the replicas only persisted the confirmed lsn of the current
synchronous transaction queue owner. As soon as the onwer changed, the
info about which lsn was confirmed by the previous owner was lost.

Actually, this info is needed to correctly filter synchro requests
coming from the old term, so start tracking confirmed vclock instead of
the confirmed lsn on replicas.

In-scope of #9138

NO_TEST=covered by the next commit
NO_CHANGELOG=internal change

@TarantoolBot document
Title: Document new IPROTO_RAFT_PROMOTE request field

IPROTO_RAFT_PROMOTE and IPROTO_RAFT_DEMOTE requests receive a new key
value pair:

IPROTO_VCLOCK : MP_MAP

The vclock holds a confirmed vclock of the node sending the request.