raft: persist vote for self together with term bump

Commit c9155ac8 ("raft: persist new term and vote separately") made
the nodes persist new term and vote separately, using 2 WAL writes.
Writing the term first is needed to flush all the ongoing transactions,
so that the node's vclock is updated and can be checked against the
candidate's vclock. Otherwise it could happen that the node persists a
vote for some candidate only to find that it's vclock would actually
become incomparable with the candidate's.

Actually, this guard is not needed when checking a vote for self,
because a node can always vote for self. Besides, splitting term bump
and vote can lead to increased probability of split-vote. It may happen
that a candidate bumps and broadcasts the new term without a vote,
making other nodes vote for self. Let's go back to writing term and vote
together for self votes.

This change makes raft candidate persist term bump and vote for self in
one WAL write instead of two, so all the tests which count WAL writes or
expect 2 separate state updates for term and vote are rewritten.

Prerequisite #8497

NO_DOC=not user-visible
NO_CHANGELOG=not user-visible

(cherry picked from commit 8a124e50)