!act,doc #3893 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. The unsubscriber is an system actor,
  and won't be stopped automagically if a bus stops to be used (hard to
  determine what "stops being used" is)
* 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`
* Will unregister from unsubscriber, when no more subscriptions for
  given subscriber are left in this bus.
* Added missing "Java API: " for some types
* Updated docs to point out the automatic subscriber purging (on terminated)