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/scala/akka/serialization/SerializeSpec.scala

@@ -88,8 +88,9 @@ object SerializationTests {
     }
   }
 
-  class FooUntypedActor extends UntypedActor {
-    def onReceive(message: Any) {}
+  class FooAbstractActor extends AbstractActor {
+    override def createReceive(): AbstractActor.Receive =
+      receiveBuilder().build()
   }
 
   class NonSerializableActor(system: ActorSystem) extends Actor {
@@ -286,7 +287,7 @@ class VerifySerializabilitySpec extends AkkaSpec(SerializationTests.verifySerial
     val a = system.actorOf(Props[FooActor])
     system stop a
 
-    val b = system.actorOf(Props(new FooActor))
+    val b = system.actorOf(Props(new FooAbstractActor))
     system stop b
 
     intercept[IllegalArgumentException] {