improve AbstractActor, #21717

* Receive class that wraps PartialFunction, to avoid
  scary scala types
* move AbstractActorContext to AbstractActor.ActorContext
* converting docs, many, many UntypedActor
* removing UntypedActor docs
* add unit test for ReceiveBuilder
* MiMa filters
* consistent use of getContext(), self(), sender()
* rename cross references
* migration guide
* skip samples for now
* improve match type safetyi, add matchUnchecked
  * the `? extends P` caused code like this to compile:
    `match(String.class, (Integer i) -> {})`
  * added matchUnchecked, since it can still be useful (um, convenient)
    to be able to do:
    `matchUnchecked(List.class, (List<String> list) -> {})`
* eleminate some scala.Option
  * preRestart
  * findChild
  * ActorIdentity.getActorRef

akka-docs/rst/java/code/docs/testkit/TestKitSampleTest.java

@@ -24,12 +24,12 @@ public class TestKitSampleTest {
     public void onReceive(Object msg) {
     
       if (msg.equals("hello")) {
-        getSender().tell("world", getSelf());
+        sender().tell("world", self());
         if (target != null) target.forward(msg, getContext());
       
       } else if (msg instanceof ActorRef) {
         target = (ActorRef) msg;
-        getSender().tell("done", getSelf());
+        sender().tell("done", self());
       }
     }
   }