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/NodeLeavingAndExitingSpec.scala

@@ -18,12 +18,8 @@ object NodeLeavingAndExitingMultiJvmSpec extends MultiNodeConfig {
   val second = role("second")
   val third = role("third")
 
-  commonConfig(
-    debugConfig(on = false)
-      .withFallback(ConfigFactory.parseString("""
-          # turn off unreachable reaper
-          akka.cluster.unreachable-nodes-reaper-interval = 300 s""")
-        .withFallback(MultiNodeClusterSpec.clusterConfigWithFailureDetectorPuppet)))
+  commonConfig(debugConfig(on = false).
+    withFallback(MultiNodeClusterSpec.clusterConfigWithFailureDetectorPuppet))
 }
 
 class NodeLeavingAndExitingMultiJvmNode1 extends NodeLeavingAndExitingSpec
@@ -63,9 +59,6 @@ abstract class NodeLeavingAndExitingSpec
         }
         enterBarrier("second-left")
 
-        val expectedAddresses = roles.toSet map address
-        awaitAssert(clusterView.members.map(_.address) must be(expectedAddresses))
-
         // Verify that 'second' node is set to EXITING
         exitingLatch.await