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/common/code/docs/circuitbreaker/DangerousJavaActor.java

@@ -5,7 +5,7 @@ package docs.circuitbreaker;
 
 //#imports1
 
-import akka.actor.UntypedActor;
+import akka.actor.AbstractActor;
 import scala.concurrent.Future;
 import akka.event.LoggingAdapter;
 import scala.concurrent.duration.Duration;
@@ -20,7 +20,7 @@ import java.util.concurrent.Callable;
 //#imports1
 
 //#circuit-breaker-initialization
-public class DangerousJavaActor extends UntypedActor {
+public class DangerousJavaActor extends AbstractActor {
 
   private final CircuitBreaker breaker;
   private final LoggingAdapter log = Logging.getLogger(getContext().system(), this);
@@ -47,22 +47,21 @@ public class DangerousJavaActor extends UntypedActor {
   }
 
   @Override
-  public void onReceive(Object message) {
-    if (message instanceof String) {
-      String m = (String) message;
-      if ("is my middle name".equals(m)) {
+  public Receive createReceive() {
+    return receiveBuilder().
+      match(String.class, m -> "is my middle name".equals(m), m -> {
         pipe(
           breaker.callWithCircuitBreaker(() -> 
             future(() -> dangerousCall(), getContext().dispatcher())
           ), getContext().dispatcher()
-        ).to(getSender());
-      }
-      if ("block for me".equals(m)) {
-        getSender().tell(breaker
+        ).to(sender());
+      })
+      .match(String.class, m -> "block for me".equals(m), m -> {
+        sender().tell(breaker
           .callWithSyncCircuitBreaker(
-            () -> dangerousCall()), getSelf());
-      }
-    }
+            () -> dangerousCall()), self());
+      })
+      .build();
   }
 //#circuit-breaker-usage