add CoordinatedShutdown, #21537

* CoordinatedShutdown that can run tasks for configured phases in order (DAG)
* coordinate handover/shutdown of singleton with cluster exiting/shutdown
* phase config obj with depends-on list
* integrate graceful leaving of sharding in coordinated shutdown
* add timeout and recover
* add some missing artery ports to tests
* leave via CoordinatedShutdown.run
* optionally exit-jvm in last phase
* run via jvm shutdown hook
* send ExitingConfirmed to leader before shutdown of Exiting
  to not have to wait for failure detector to mark it as
  unreachable before removing
* the unreachable signal is still kept as a safe guard if
  message is lost or leader dies
* PhaseClusterExiting vs MemberExited in ClusterSingletonManager
* terminate ActorSystem when cluster shutdown (via Down)
* add more predefined and custom phases
* reference documentation
* migration guide
* problem when the leader order was sys2, sys1, sys3,
  then sys3 could not perform it's duties and move Leving sys1 to
  Exiting because it was observing sys1 as unreachable
* exclude Leaving with exitingConfirmed from convergence condidtion

akka-cluster-tools/src/multi-jvm/scala/akka/cluster/client/ClusterClientSpec.scala

@@ -453,8 +453,10 @@ class ClusterClientSpec extends MultiNodeSpec(ClusterClientSpec) with STMultiNod
         val sys2 = ActorSystem(
           system.name,
           ConfigFactory.parseString(
-            if (RARP(system).provider.remoteSettings.Artery.Enabled) s"akka.remote.artery.canonical.port=$port"
-            else s"akka.remote.netty.tcp.port=$port").withFallback(system.settings.config))
+            s"""
+              akka.remote.artery.canonical.port=$port
+              akka.remote.netty.tcp.port=$port
+              """).withFallback(system.settings.config))
         Cluster(sys2).join(Cluster(sys2).selfAddress)
         val service2 = sys2.actorOf(Props(classOf[TestService], testActor), "service2")
         ClusterClientReceptionist(sys2).registerService(service2)