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)
* I have been thinking hard why the old leader didn't receive
  a proper LeaderChanged earlier, when changing status from
  leaving -> exiting, but I can't really explain why that was not
  published or handled. I might create a separate ticket for that.