!act,doc #3893 Removed isTerminated checks from ActorClassification

Instead of `isTerminated` we now use death watch on subscribers in `ActorClassification`

This PR is a follow up from https://github.com/akka/akka/pull/2011#issuecomment-35864011 where `isTerminated` has been replaced with death watching actors.

Suggested to be pulled in for 2.4, since it's breaking API (`ActorClassification` now requires an ActorSystem, and it's required to stop the system actor ("unwatcher"), when done using such an event bus).
Or can be rewritten to use init method based approach instead.