* 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
* Renamed isRunning to isTerminated (with negation of course)
* Removed Running from JMX API, since the mbean is deregistered anyway
* Cleanup isAvailable, isUnavailbe
* Misc minor
* MBeanSpec
* Added Members and Unreachable to JMX API
* Removed Convergence from JMX API, because it will
not be exposed when ticket #2692 is merged
* Updated documentation and akka-cluster script
