Medium: raft: don't ignore duplicate vote_for packets

Which are sometimes not duplicate, but from new elections. The
term's not increased in case there was no tie, and the candidate
expects others to keep their votes.
Dev: test: add slow start test
Dev: ticket: keep processing if the next state is follower
Dev: test: improve the simultaneous start test
Dev: remove some debugging
Dev: update one log message
Medium: config: don't change ticket defaults in ticket stanzas (use __defaults__)
Medium: config: don't allow too long packet retry period
Low: main: log something on exit
Medium: raft: ack revokes (bnc#882057)

All servers must ack the revoke operation sent by the leader.
If an ack is missing, the packet is being resent, as with
heartbeats. This makes revokes more robust.

This commit introduces a new RPC: "Ack." This RPC is also used to
acknowledge heartbeats.
Dev: raft: reseting leader in elections is obsolete
Dev: test: add a grant with no arbitrators test
Dev: more detailed debugging messages
Dev: raft: remove unneeded test from ticket update
Dev: more debugging
Dev: test: improve listing ticket owner
Dev: test: add the packet loss test
Medium: raft: always accept heartbeats and updates from a valid leader (bnc#882209)

We need to trust the ticket leader.

On rejected elections, it may happen that some members remain
with a (previously rejected) higher term. They need then to
accept the valid lower term which is still at the leader. This
could happen on a say 2-3 split brain or high packet loss
network.
Medium: raft: ignore VOTE_FOR if we lost the candidate status (bnc#882209)

A server expects VOTE_FOR only if it's a candidate or otherwise
the leader wants to step down. Rearrange processing accordingly.

This can happen if REQ_VOTE/VOTE_FOR packets cross and REQ_VOTE
was in between rejected by another server.
Dev: test: dump the configuration file to the logfile