=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;
+sam #13890 Add Akka Main Activator template for Java 8
!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
* backing structure for ActorClassification swaped from
  ConcurrentHashMap to immutable.Map with CAS operations on it. This is
  required to avoid races and guarantee register/unregister ordering
  (messages sent with proper sequence numbers) to the unsubscriber.
  Performance tested it and still above 1.3million subscribe+unsubscribe
  ops per second (mac i7, retina), where as the CHM version was
  4 million - but that one could only work in the presence of
  itTerminated - so we pay the price here for removing it.
* `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)
Merge pull request #15054 from akka/wip-13890-akka-java8-template-master-ban

(Forward Port) +sam #13890 Add Akka Main Activator template for Java 8
Merge pull request #15024 from ktoso/wip-removing-isTerminated-from-buses-ktoso

!act,doc Removing isTerminated from EventStream and ActorClassification
=pro #15031 use sbt-unidoc plugin