=str #17200 Stop shard region when MemberRemoved

Two issues:

1) ShardRegion actor must stop itself when the node is shutting down,
   ie. when receiving MemberRemoved(selfAddress)
2) ShardCoordinator must not persist anything when the node is shutting
   down. MemberRemoved of other shard regions will trigger Terminated,
   which must not be persisted, because then the next coordinator will
   replay those events and end up in wrong state. This is a problem
   announced itself when using leaving as illustrated in the new test.

To solve the second issue I have added a new ClusterShuttingDown event
that is published before the MemberRemoved events. Note that Terminated
is triggered by MemberRemoved.

(cherry picked from commit 1b272c72597beece9d93f0054f4b58e3d25f9ae2)

akka-cluster/src/main/scala/akka/cluster/ClusterEvent.scala

@@ -171,6 +171,17 @@ object ClusterEvent {
     def getLeader: Address = leader orNull
   }
 
+  /**
+   * This event is published when the cluster node is shutting down,
+   * before the final [[MemberRemoved]] events are published.
+   */
+  final case object ClusterShuttingDown extends ClusterDomainEvent
+
+  /**
+   * Java API: get the singleton instance of [[ClusterShuttingDown]] event
+   */
+  def getClusterShuttingDownInstance = ClusterShuttingDown
+
   /**
    * Marker interface to facilitate subscription of
    * both [[UnreachableMember]] and [[ReachableMember]].
@@ -328,6 +339,7 @@ private[cluster] final class ClusterDomainEventPublisher extends Actor with Acto
 
   override def postStop(): Unit = {
     // publish the final removed state before shutting down
+    publish(ClusterShuttingDown)
     publishChanges(Gossip.empty)
   }