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
Remove LargeClusterSpec, superseded by StressSpec, see #2786
Merge pull request #940 from akka/wip-2786-cluster-stress-patriknw

Stress / long running test of cluster, see #2786