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/ParentChildTest.java

@@ -28,7 +28,7 @@ public class ParentChildTest {
 
   //#test-example
   static class Parent extends UntypedActor {
-    final ActorRef child = context().actorOf(Props.create(Child.class), "child");
+    final ActorRef child = getContext().actorOf(Props.create(Child.class), "child");
     boolean ponged = false;
 
     @Override public void onReceive(Object message) throws Exception {
@@ -45,7 +45,7 @@ public class ParentChildTest {
   static class Child extends UntypedActor {
     @Override public void onReceive(Object message) throws Exception {
       if ("ping".equals(message)) {
-        context().parent().tell("pong", self());
+        getContext().parent().tell("pong", self());
       } else {
         unhandled(message);
       }
@@ -79,7 +79,7 @@ public class ParentChildTest {
     boolean ponged = false;
 
     public DependentParent(Props childProps) {
-      child = context().actorOf(childProps, "child");
+      child = getContext().actorOf(childProps, "child");
     }
 
     @Override public void onReceive(Object message) throws Exception {
@@ -102,7 +102,7 @@ public class ParentChildTest {
 
     public GenericDependentParent(Function<ActorRefFactory, ActorRef> childMaker)
       throws Exception {
-      child = childMaker.apply(context());
+      child = childMaker.apply(getContext());
     }
 
     @Override public void onReceive(Object message) throws Exception {
@@ -175,13 +175,13 @@ public class ParentChildTest {
 
     @Override public Actor create() throws Exception {
       return new UntypedActor() {
-        final ActorRef child = context().actorOf(Props.create(Child.class), "child");
+        final ActorRef child = getContext().actorOf(Props.create(Child.class), "child");
 
         @Override public void onReceive(Object x) throws Exception {
           if (sender().equals(child)) {
-            proxy.ref().forward(x, context());
+            proxy.ref().forward(x, getContext());
           } else {
-            child.forward(x, context());
+            child.forward(x, getContext());
           }
         }
       };

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

@@ -37,7 +37,7 @@ public class TestKitDocTest {
   static class MyActor extends UntypedActor {
     public void onReceive(Object o) throws Exception {
       if (o.equals("say42")) {
-        getSender().tell(42, getSelf());
+        sender().tell(42, self());
       } else if (o instanceof Exception) {
         throw (Exception) o;
       }

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());
       }
     }
   }