Dev: raft: separate INIT and FOLLOWER states
Dev: ticket: rename client ticket operations
Dev: raft: add revoke operation
Medium: raft: on vote request reply to sender only
Dev: raft: delay setting leader on vote request
High: raft: elections not allowed if the term's still valid (bnc#876300)

If we the term is still valid, don't let others start the
election. It may happen that a follower goes disconnected,
considers the ticket lost after a while and starts election, then
comes back and tries to get the ticket which is still valid and
supported by the majority. The leader, if it loses majority, will
lose the ticket itself and agree to do new elections.
Medium: raft: accept valid heartbeats in the candidate state