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/future/FutureDocTest.java

@@ -65,7 +65,7 @@ import org.junit.Test;
 import akka.testkit.AkkaSpec;
 import akka.actor.Status.Failure;
 import akka.actor.ActorSystem;
-import akka.actor.UntypedActor;
+import akka.actor.AbstractActor;
 import akka.actor.ActorRef;
 import akka.actor.Props;
 import akka.pattern.Patterns;
@@ -656,20 +656,21 @@ public class FutureDocTest extends AbstractJavaTest {
 
 
 
-  public static class MyActor extends UntypedActor {
-    public void onReceive(Object message) {
-      if (message instanceof String) {
-        getSender().tell(((String) message).toUpperCase(), getSelf());
-      } else if (message instanceof Integer) {
-        int i = ((Integer) message).intValue();
-        if (i < 0) {
-          getSender().tell(new Failure(new ArithmeticException("Negative values not supported")), getSelf());
-        } else {
-          getSender().tell(i, getSelf());
-        }
-      } else {
-        unhandled(message);
-      }
+  public static class MyActor extends AbstractActor {
+    @Override
+    public Receive createReceive() {
+      return receiveBuilder()
+        .match(String.class, msg -> {
+          sender().tell(msg.toUpperCase(), self());
+        })
+        .match(Integer.class, i -> {
+          if (i < 0) {
+            sender().tell(new Failure(new ArithmeticException("Negative values not supported")), self());
+          } else {
+            sender().tell(i, self());
+          }
+        })
+        .build();
     }
   }
 }