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/actorlambda/MyStoppingActor.java

@@ -0,0 +1,29 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package docs.actorlambda;
+
+//#my-stopping-actor
+import akka.actor.ActorRef;
+import akka.actor.AbstractActor;
+
+public class MyStoppingActor extends AbstractActor {
+
+  ActorRef child = null;
+
+  // ... creation of child ...
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+      .matchEquals("interrupt-child", m ->
+        getContext().stop(child)
+      )
+      .matchEquals("done", m ->
+        getContext().stop(self())
+      )
+      .build();
+  }
+}
+//#my-stopping-actor
+