Stress / long running test of cluster, see #2786

* akka.cluster.StressSpec
* Configurable number of nodes and duration for each step
* Report metrics and phi periodically to see progress
* Configurable payload size
* Test of various join and remove scenarios
* Test of watch
* Exercise supervision
* Report cluster stats
* Test with many actors in tree structure

Apart from the test this commit also solves some issues:

* Avoid adding back members when downed in ClusterHeartbeatSender
* Avoid duplicate close of ClusterReadView
* Add back the publish of AddressTerminated when MemberDowned/Removed
  it was lost in merge of "publish on convergence", see #2779

akka-cluster/src/multi-jvm/scala/akka/cluster/MBeanSpec.scala

@@ -115,7 +115,7 @@ abstract class MBeanSpec
       enterBarrier("fourth-down")
 
       runOn(first, second, third) {
-        awaitUpConvergence(3, canNotBePartOfMemberRing = List(fourthAddress))
+        awaitUpConvergence(3, canNotBePartOfMemberRing = Set(fourthAddress))
         assertMembers(clusterView.members, first, second, third)
       }