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/pattern/SchedulerPatternTest.java

@@ -30,7 +30,7 @@ public class SchedulerPatternTest extends AbstractJavaTest {
     private final Cancellable tick = getContext().system().scheduler().schedule(
       Duration.create(500, TimeUnit.MILLISECONDS),
       Duration.create(1, TimeUnit.SECONDS),
-      getSelf(), "tick", getContext().dispatcher(), null);
+      self(), "tick", getContext().dispatcher(), null);
     //#schedule-constructor
     // this variable and constructor is declared here to not show up in the docs
     final ActorRef target;
@@ -49,7 +49,7 @@ public class SchedulerPatternTest extends AbstractJavaTest {
       if (message.equals("tick")) {
         // do something useful here
         //#schedule-constructor
-        target.tell(message, getSelf());
+        target.tell(message, self());
         //#schedule-constructor
       }
       //#schedule-constructor
@@ -79,7 +79,7 @@ public class SchedulerPatternTest extends AbstractJavaTest {
     public void preStart() {
       getContext().system().scheduler().scheduleOnce(
         Duration.create(500, TimeUnit.MILLISECONDS),
-        getSelf(), "tick", getContext().dispatcher(), null);
+        self(), "tick", getContext().dispatcher(), null);
     }
 
     // override postRestart so we don't call preStart and schedule a new message
@@ -93,10 +93,10 @@ public class SchedulerPatternTest extends AbstractJavaTest {
         // send another periodic tick after the specified delay
         getContext().system().scheduler().scheduleOnce(
           Duration.create(1, TimeUnit.SECONDS),
-          getSelf(), "tick", getContext().dispatcher(), null);
+          self(), "tick", getContext().dispatcher(), null);
         // do something useful here
         //#schedule-receive
-        target.tell(message, getSelf());
+        target.tell(message, self());
         //#schedule-receive
       }
       //#schedule-receive