Hardening of another corner case in cluster singleton, see #3017

* It was an unlikely situatation that was not covered,
  the new leader didn't know previous, because it transitioned
  from Start -> BecomeLeader, old leader was removed and got
  LeaderChanged(None), so none of them could request the other
  for hand-over or take-over.
* Taken care of with the retry timeouts, also when leader
  receives LeaderChanged(None)
* This also revealed a flaw in how LeaderChanged events are published.
  The leader should have received a proper LeaderChanged earlier, when 
  changing status from leaving -> exiting. The leader events must also be
  buffered, similar to member events. Otherwise all changes might not be
  published because it doesn't have to be convergence on *all* nodes between
  transitions.