This series implements leader stepdown extension. See patch 4 for
justification for its existence. First three patches either implement
cleanups to existing code that future patch will touch or fix bugs
that need to be fixed in order for stepdown test to work.
* 'raft-leader-stepdown-v3' of github.com:scylladb/scylla-dev:
raft: add test for leader stepdown
raft: introduce leader stepdown procedure
raft: fix replication when leader is not part of current config
raft: do not update last election time if current leader is not a part of current configuration
raft: move log limiting semaphore into the leader state
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