+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-persistence-java/src/main/java/sample/persistence/PersistentActorExample.java

@@ -127,6 +127,6 @@ public class PersistentActorExample {
         persistentActor.tell("print", null);
 
         Thread.sleep(1000);
-        system.shutdown();
+        system.terminate();
     }
 }

akka-samples/akka-sample-persistence-java/src/main/java/sample/persistence/SnapshotExample.java

@@ -95,6 +95,6 @@ public class SnapshotExample {
         persistentActor.tell("print", null);
 
         Thread.sleep(1000);
-        system.shutdown();
+        system.terminate();
     }
 }