!clu #2307 Allow transition from unreachable to reachable

* Replace unreachable Set with Reachability table
* Unreachable members stay in member Set
* Downing a live member was moved it to the unreachable Set,
  and then removed from there by the leader. That will not
  work when flipping back to reachable, so a Down member must
  be detected as unreachable before beeing removed. Similar
  to Exiting. Member shuts down itself if it sees itself as
  Down.
* Flip back to reachable when failure detector monitors it as
  available again
* ReachableMember event
* Can't ignore gossip from aggregated unreachable (see SurviveNetworkInstabilitySpec)
* Make use of ReachableMember event in cluster router
* End heartbeat when acknowledged, EndHeartbeatAck
* Remove nr-of-end-heartbeats from conf
* Full reachability info in JMX cluster status
* Don't use interval after unreachable for AccrualFailureDetector history
* Add QuarantinedEvent to remoting, used for Reachability.Terminated
* Prune reachability table when all reachable
* Update documentation
* Performance testing and optimizations

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

@@ -109,7 +109,7 @@ private[cluster] object StressMultiJvmSpec extends MultiNodeConfig {
       # by tree-width (number of children for each actor) and
       # tree-levels, total number of actors can be calculated by
       # (width * math.pow(width, levels) - 1) / (width - 1)
-      tree-width = 5
+      tree-width = 4
       tree-levels = 4
       report-metrics-interval = 10s
       # scale convergence within timeouts with this factor