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
Merge pull request #1384 from akka/wip-3279-gossip-status-patriknw

Reduce amount of gossip data transferred in idle cluster, see #3279