* Otherwise some changes might never be published, since it doesn't have
to be convergence on all nodes inbetween all transitions.
* Detected by a failure ClusterSingletonManagerSpec.
* Added a test to simulate the failure scenario.
* 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)
* The old leader should have received a propert LeaderChanged
earlier, which is a flaw in the way we publish leader events.
That part will be fixed in a separate commit.
* ClusterSingletonManager
* ClusterSingletonManagerSpec multi-node test
* Use in cluster router with single master sample
* Extensive logging to be able to understand what is
going on
* Java api
* Add cluster dependency to contrib
* Add contrib dependency to sample
* Scaladoc
* rst docs in contrib area, ref from cluster docs
