+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-docs-java-lambda/src/test/java/docs/actor/InitializationDocTest.java

@@ -10,11 +10,11 @@ import akka.testkit.JavaTestKit;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
-import scala.PartialFunction;
 import scala.concurrent.duration.Duration;
-import scala.runtime.BoxedUnit;
+import scala.concurrent.Await;
 
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 
 public class InitializationDocTest {
 
@@ -26,9 +26,8 @@ public class InitializationDocTest {
   }
 
   @AfterClass
-  public static void afterClass() {
-    system.shutdown();
-    system.awaitTermination(Duration.create("5 seconds"));
+  public static void afterClass() throws Exception {
+      Await.ready(system.terminate(), Duration.create("5 seconds"));
   }
 
   public static class MessageInitExample extends AbstractActor {