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/camel/Consumer2.java

@@ -12,7 +12,7 @@ public class Consumer2 extends UntypedConsumerActor {
     if (message instanceof CamelMessage) {
       CamelMessage camelMessage = (CamelMessage) message;
       String body = camelMessage.getBodyAs(String.class, getCamelContext());
-      getSender().tell(String.format("Received message: %s",body), getSelf());
+      sender().tell(String.format("Received message: %s",body), self());
     } else
       unhandled(message);
   }

akka-docs/rst/java/code/docs/camel/Consumer3.java

@@ -18,12 +18,12 @@ public class Consumer3 extends UntypedConsumerActor{
 
   public void onReceive(Object message) {
     if (message instanceof CamelMessage) {
-      getSender().tell(Ack.getInstance(), getSelf());
+      sender().tell(Ack.getInstance(), self());
       // on success
       // ..
       Exception someException = new Exception("e1");
       // on failure
-      getSender().tell(new Status.Failure(someException), getSelf());
+      sender().tell(new Status.Failure(someException), self());
     } else
       unhandled(message);
   }

akka-docs/rst/java/code/docs/camel/Consumer4.java

@@ -24,7 +24,7 @@ public class Consumer4 extends UntypedConsumerActor {
     if (message instanceof CamelMessage) {
       CamelMessage camelMessage = (CamelMessage) message;
       String body = camelMessage.getBodyAs(String.class, getCamelContext());
-      getSender().tell(String.format("Hello %s",body), getSelf());
+      sender().tell(String.format("Hello %s",body), self());
     } else
       unhandled(message);
   }

akka-docs/rst/java/code/docs/camel/ErrorThrowingConsumer.java

@@ -47,7 +47,7 @@ public class ErrorThrowingConsumer extends UntypedConsumerActor{
 
   @Override
   public void preRestart(Throwable reason, Option<Object> message) {
-    getSender().tell(new Status.Failure(reason), getSelf());
+    sender().tell(new Status.Failure(reason), self());
   }
 }
 //#ErrorThrowingConsumer

akka-docs/rst/java/code/docs/camel/MyActor.java

@@ -1,11 +1,11 @@
 package docs.camel;
 //#ProducerTemplate
-import akka.actor.UntypedActor;
+import akka.actor.UntypedAbstractActor;
 import akka.camel.Camel;
 import akka.camel.CamelExtension;
 import org.apache.camel.ProducerTemplate;
 
-public class MyActor extends UntypedActor {
+public class MyActor extends UntypedAbstractActor {
   public void onReceive(Object message) {
     Camel camel = CamelExtension.get(getContext().system());
     ProducerTemplate template = camel.template();

akka-docs/rst/java/code/docs/camel/RequestBodyActor.java

@@ -1,15 +1,20 @@
 package docs.camel;
 //#RequestProducerTemplate
-import akka.actor.UntypedActor;
+import akka.actor.AbstractActor;
 import akka.camel.Camel;
 import akka.camel.CamelExtension;
 import org.apache.camel.ProducerTemplate;
 
-public class RequestBodyActor extends UntypedActor {
-  public void onReceive(Object message) {
-    Camel camel = CamelExtension.get(getContext().system());
-    ProducerTemplate template = camel.template();
-    getSender().tell(template.requestBody("direct:news", message), getSelf());
+public class RequestBodyActor extends AbstractActor {
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+      .matchAny(message -> {
+        Camel camel = CamelExtension.get(getContext().system());
+        ProducerTemplate template = camel.template();
+        sender().tell(template.requestBody("direct:news", message), self());
+      })
+      .build();
   }
 }
 //#RequestProducerTemplate

akka-docs/rst/java/code/docs/camel/Responder.java

@@ -1,16 +1,16 @@
 package docs.camel;
 //#CustomRoute
-import akka.actor.UntypedActor;
+import akka.actor.UntypedAbstractActor;
 import akka.camel.CamelMessage;
 import akka.dispatch.Mapper;
 import akka.japi.Function;
 
-public class Responder extends UntypedActor{
+public class Responder extends UntypedAbstractActor{
 
   public void onReceive(Object message) {
     if (message instanceof CamelMessage) {
       CamelMessage camelMessage = (CamelMessage) message;
-      getSender().tell(createResponse(camelMessage), getSelf());
+      sender().tell(createResponse(camelMessage), self());
     } else
       unhandled(message);
   }

akka-docs/rst/java/code/docs/camel/ResponseReceiver.java

@@ -1,9 +1,9 @@
 package docs.camel;
 //#RouteResponse
-import akka.actor.UntypedActor;
+import akka.actor.UntypedAbstractActor;
 import akka.camel.CamelMessage;
 
-public class ResponseReceiver extends UntypedActor{
+public class ResponseReceiver extends UntypedAbstractActor{
   public void onReceive(Object message) {
      if(message instanceof CamelMessage) {
        // do something with the forwarded response