clean up DeathWatch semantics, see #3222

- DeathPactException => Stop in defaultStrategy
- ensure that after calling `context unwatch ref` we will not process a
  Terminated(`ref`) anymore, even if it was already enqueued (i.e.
  unwatch() happens between DeathWatchNotification and Terminated)

akka-docs/rst/scala/actors.rst

@@ -310,9 +310,9 @@ monitoring of an already terminated actor leads to the immediate generation of
 the :class:`Terminated` message.
 
 It is also possible to deregister from watching another actor’s liveliness
-using ``context.unwatch(target)``, but obviously this cannot guarantee
-non-reception of the :class:`Terminated` message because that may already have
-been queued.
+using ``context.unwatch(target)``. This works even if the :class:`Terminated`
+message has already been enqueued in the mailbox; after calling :meth:`unwatch`
+no :class:`Terminated` message for that actor will be processed anymore.
 
 Start Hook
 ----------