!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/ConvergenceSpec.scala

@@ -70,12 +70,11 @@ abstract class ConvergenceSpec(multiNodeConfig: ConvergenceMultiNodeConfig)
         within(28 seconds) {
           // third becomes unreachable
           awaitAssert(clusterView.unreachableMembers.size must be(1))
-          awaitAssert(clusterView.members.size must be(2))
-          awaitAssert(clusterView.members.map(_.status) must be(Set(MemberStatus.Up)))
           awaitSeenSameState(first, second)
           // still one unreachable
           clusterView.unreachableMembers.size must be(1)
           clusterView.unreachableMembers.head.address must be(thirdAddress)
+          clusterView.members.size must be(3)
 
         }
       }
@@ -96,7 +95,7 @@ abstract class ConvergenceSpec(multiNodeConfig: ConvergenceMultiNodeConfig)
 
       runOn(first, second, fourth) {
         for (n ← 1 to 5) {
-          awaitAssert(clusterView.members.size must be(2))
+          awaitAssert(clusterView.members.size must be(3))
           awaitSeenSameState(first, second, fourth)
           memberStatus(first) must be(Some(MemberStatus.Up))
           memberStatus(second) must be(Some(MemberStatus.Up))