=act #3623 unsubscribe when actors terminate, instead of `isTerminated`

* Moved to removing actors proactively when they are terminated instead of
  checking `isTerminated` during publish.
* Subscribers which have registered before initializing the unsubscriber
  will be aggregated in a Seq until one is registered and then it will
  take responsibility of unregistering them on termination.
* Initialization of the unsubscriber can only be run once - attempting
  to initialize the event stream with another unsubscriber will fail,
  and init will return false.
* Assumed having an init (mutable) method on the `EventBus` is fine, as
  it has such methods already and @patriknw's comment in the task for
  this.
* 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.
* Evacuated the expensive `hasSubscription` call out of eventstream's `unsubscribe` call, and instead making the Unsubscriber check this before it stops watching an actor. If in the mean time the same actor got subscribed, there will be a new Subscribe message emited - so we're good on that side. Also, if it would terminate before the unsubscriber gets the Register message it will call `watch(actor)` on a dead actor, which results in getting Terminated for it, thus we'll stop watching it from the Unsubscriber as expected.

Final squash and small cleanup. Please review again;