raft: fix crash on election_timeout reconfig

It used to crash if done during election on a node voted for
anybody, it is a candidate, it doesn't know a leader yet, but has
a WAL write in progress.

Thus it could only happen if the term was bumped by a message from
a non-leader node and wasn't flushed to the disk yet.

The patch makes the reconfig check if there is a WAL write in
progress. Then don't do anything.

Could also check for volatile vote instead of persistent, but it
would create the same problem for the case when started writing
vote for self and didn't finish yet. Reconfig would crash.