+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/src/main/java/sample/hello/Main2.java

@@ -30,7 +30,7 @@ public class Main2 {
     public void onReceive(Object msg) {
       if (msg instanceof Terminated) {
         log.info("{} has terminated, shutting down system", ref.path());
-        getContext().system().shutdown();
+        getContext().system().terminate();
       } else {
         unhandled(msg);
       }