flush messages on shutdown, #20811

* StreamSupervisor as system actor so that it is
  stopped after ordinary actors
* when transport is shutdown send flush message to all
  outbound associations (over control stream) and wait for ack
  or timeout

akka-stream/src/main/scala/akka/stream/ActorMaterializer.scala

@@ -79,6 +79,22 @@ object ActorMaterializer {
   def apply(materializerSettings: ActorMaterializerSettings)(implicit context: ActorRefFactory): ActorMaterializer =
     apply(Some(materializerSettings), None)
 
+  /**
+   * INTERNAL API: Creates the `StreamSupervisor` as a system actor.
+   */
+  private[akka] def systemMaterializer(materializerSettings: ActorMaterializerSettings, namePrefix: String,
+                                       system: ExtendedActorSystem): ActorMaterializer = {
+    val haveShutDown = new AtomicBoolean(false)
+    new ActorMaterializerImpl(
+      system,
+      materializerSettings,
+      system.dispatchers,
+      system.systemActorOf(StreamSupervisor.props(materializerSettings, haveShutDown)
+        .withDispatcher(materializerSettings.dispatcher), StreamSupervisor.nextName()),
+      haveShutDown,
+      FlowNames(system).name.copy(namePrefix))
+  }
+
   /**
    * Java API: Creates a ActorMaterializer which will execute every step of a transformation
    * pipeline within its own [[akka.actor.Actor]]. The required [[akka.actor.ActorRefFactory]]