Coordinate shutdown of ClusterDeathWatchSpec with messages, see #3255

* Added MultiNodeClusterSpec.EndActor for this purpose
* Changed UnreachableNodeJoinsAgainSpec to use the same
Merge pull request #1365 from akka/wip-3255-ClusterDeathWatchSpec-patriknw

Coordinate shutdown of ClusterDeathWatchSpec with messages, see #3255