+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-supervision-java-lambda/src/main/java/supervision/Main.java

@@ -33,7 +33,6 @@ public class Main {
     Integer result = Await.result(ask(calculatorService, task, new Timeout(duration)).mapTo(classTag(Integer.class)), duration);
     System.out.println("Got result: " + result);
 
-    system.shutdown();
-    system.awaitTermination();
+    Await.ready(system.terminate(), Duration.Inf());
   }
 }