+act - 15757 - Reworks implementation of ActorSystem shutdown

* deprecates awaitTermination, shutdown and isTerminated
  * introduces a terminate-method that returns a Future[Unit]
  * introduces a whenTerminated-method that returns a Future[Unit]
  * simplifies the implementation by removing blocking constructs
  * adds tests for terminate() and whenTerminated

akka-samples/akka-sample-main-java-lambda/src/main/java/sample/hello/Main2.java

@@ -21,7 +21,7 @@ public class Main2 {
       receive(ReceiveBuilder.
         match(Terminated.class, t -> {
           log().info("{} has terminated, shutting down system", ref.path());
-          context().system().shutdown();
+          context().system().terminate();
         }).build());
     }
   }