raft: introduce on_update trigger

Raft state machine now has a trigger invoked each time when any of
the visible Raft attributes is changed: state, term, vote.

The trigger is needed to commit synchronous transactions of an old
leader, when a new leader is elected. This is done via a trigger
so as not to depend on box in raft code too much. That would make
it harder to extract it into a new module later.

The trigger is executed in the Raft worker fiber, so as not to
stop the state machine transitions anywhere, which currently don't
contain a single yield. And the synchronous transaction queue
clearance requires a yield, to write CONFIRM and ROLLBACK records
to WAL.

Part of #5339