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-actor-tests/src/test/java/akka/japi/pf/PFBuilderTest.java

@@ -10,6 +10,7 @@ import scala.PartialFunction;
 
 import static org.junit.Assert.*;
 
+@SuppressWarnings("serial")
 public class PFBuilderTest extends JUnitSuite {
 
   @Test
@@ -18,7 +19,7 @@ public class PFBuilderTest extends JUnitSuite {
       .matchEquals("hello", s -> 1)
       .matchAny(s -> Integer.valueOf(s))
       .build();
-      
+
     assertTrue(pf.isDefinedAt("hello"));
     assertTrue(pf.isDefinedAt("42"));
     assertEquals(42, pf.apply("42").intValue());