Speed up cluster gossip processing. See #3441

Check VectorClock for common case first and cache hashCodes. See #3441
Make ClusterDaemon a bit more testable. See #3441
Changing VectorClock and GossipOverview to TreeMaps. See #3441
Make VectorClock private[cluster] and remove unused code. See #3441
Merge pull request #1535 from akka/wip-3441-speed-up-cluster-gossip-processing-ban

Speed up cluster gossip processing #3441