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

@@ -125,6 +125,6 @@ public class PersistentActorExample {
         processor.tell("print", null);
 
         Thread.sleep(1000);
-        system.shutdown();
+        system.terminate();
     }
 }

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

@@ -10,8 +10,6 @@ import akka.actor.Props;
 import akka.japi.pf.ReceiveBuilder;
 import akka.persistence.AbstractPersistentActor;
 import akka.persistence.SnapshotOffer;
-import scala.PartialFunction;
-import scala.runtime.BoxedUnit;
 
 import java.io.Serializable;
 import java.util.ArrayList;
@@ -88,6 +86,6 @@ public class SnapshotExample {
     persistentActor.tell("print", null);
 
     Thread.sleep(1000);
-    system.shutdown();
+    system.terminate();
   }
 }