=act #13894 cancel StateTimeout when FSM actor is stopped

Fix for #13894.
This does not verify what you've fixed in this PR.
The problem is that once the actor is stopped, the message still would be sent to it - resulting in a "dead letter", like so:

```
cala> MyApp.main(Array())
[INFO] [11/26/2014 12:44:03.018] [TestSystem-akka.actor.default-dispatcher-2] [akka://TestSystem/user/$a] Received message 'Some message'
[INFO] [11/26/2014 12:44:08.037] [TestSystem-akka.actor.default-dispatcher-3] [akka://TestSystem/user/$a] Message [akka.actor.FSM$TimeoutMarker] from Actor[akka://TestSystem/user/$a#1611577448] to Actor[akka://TestSystem/user/$a#1611577448] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
```

Your test should instead subscribe on the system.eventStream and check that there was no dead letters:

```scala
  val probe = TestProbe()
  system.eventStream.subscribe(probe, classOf[DeadLetter])
  probe.expectNoMsg(200.millis)
```