replication: fix extraneous split-brain alerting

Current split-brain detector implementation raises an error each time a
CONFIRM or ROLLBACK entry is received from the previous synchronous
transaction queue owner. It is assumed that the new queue owner must
have witnessed all the previous CONFIRMS. Besides, according to Raft,
ROLLBACK should never happen.

Actually there is a case when a CONFIRM from an old term is legal: it's
possible that during leader transition old leader writes a CONFIRM for
the same transaction that is confirmed by the new leader's PROMOTE. If
PROMOTE and CONFIRM lsns match there is nothing bad about such
situation.

Symmetrically, when an old leader issues a ROLLBACK with the lsn right
after the new leader's PROMOTE lsn, it is not a split-brain.

Allow such cases by tracking the last confirmed lsn for each synchronous
transaction queue owner and silently nopifying CONFIRMs with an lsn less
than the one recorded and ROLLBACKs with lsn greater than that.

Closes #9138

NO_DOC=bugfix