=act #3623 remove racy assertion, not really required in this test
=act #3623 proposed impl for unsubscribing, when last channel subsc removed

* since we must check if the subscriber has any subscribed channels left
  we had to expose this detail from SubchannelClassification via
  `hasSubscriptions`. Increases cost of ubsubscribe(actor, channel) a bit.

Please review, @patriknw. Do you think adding this method in the
classification is justified by this need? Made it private[akka], don't
think many end-users would need this information.
fix - type Event must be E
!act,doc 3623 Removed isTerminated checks from ActorClassification

Instead of isTerminated we now use death watch on subscribers.

! Breaking change - ActorClassification based event buses now require
  and actor system. Previously no actors were involved, but now someone
  has to `watch` the subscribers. If this change is not acceptable, we
  can move to an `init` method based interfacem which is less safe but
  wouldn't break API of `ActorEventBus`
* replaced isTerminated checks with watching actors
* `ActorClassification` starts the unsubscriber instance by itself,
  the unsubscriber is an system actor, and can be stopped via
  `ActorClassification#shutdown`

> unwatching could be improved I think