Hardening of cluster member leaving path, see #3309

* Removed leader commands for Shutdown and Exit
* Member shutdown itself  when it sees itself as Exiting
* Singleton cluster with status Exiting will shutdown itself,
  in case the Exiting gossip never arrives
* Exiting member not part convergence check
* Exiting member is removed by leader (on convergence) when the
  exiting member is in the unreachable set, i.e. sucessfully shutdown
* Reverted the change made for #3266, i.e. Exiting is
  detected as unreachable again.
* Adjust ClusterSingletonManager to new Exiting behaviour
* Fix bug in HeartbeatSender, which caused it to continue to
  send heartbeats to removed nodes, instead of rebalancing
* Refactoring of leaderActions method
* Leaving section in docs

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

@@ -50,6 +50,7 @@ abstract class ClusterMetricsSpec extends MultiNodeSpec(ClusterMetricsMultiJvmSp
       }
       enterBarrier("first-left")
       runOn(second, third, fourth, fifth) {
+        markNodeAsUnavailable(first)
         awaitAssert(clusterView.clusterMetrics.size must be(roles.size - 1))
       }
       enterBarrier("finished")