Enable usage of MultiJvm nrOfNodes in cluster StressSpec, see #2787

* Adjustments to StressSpec for testing large clusters
* Performance improvement of mute deadLetters
Quick fix for unreachable exiting, see #3266
Additional tests of ClusterMessageSerializer
Reduce amount of gossip data transferred in idle cluster, see #3279

* When seen same the gossip chat is initated with GossipStatus
  message containing the vclock only
* Remove conversation flag in GossipEnvelope
* Ordinary tell instead of actorSelection when replying
Move barriers of conditional tests in StressSpec, see #2787
Merge pull request #1381 from akka/wip-2787-cluster-stress-patriknw

Prepare Cluster StressSpec for heavy testing, see #2787