There are situations where a node outside the current configuration is
the only node that can become a leader. We become candidates in such
cases. But there is an easy check for when we don't need to; a comment was
added explaining that.
* kbr/candidate-outside-config-v3:
raft: sometimes become a candidate even if outside the configuration
raft: fsm: update _commit_idx when applying snapshot
e115fce8f7