!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)

akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala

@@ -84,16 +84,16 @@ trait PersistenceDocSpec {
       }
       //#deletion
     }
-    
+
     class MyProcessor4 extends Processor {
       //#recovery-completed
       override def preStart(): Unit = {
         super.preStart()
         self ! "FIRST"
       }
-      
+
       def receive = initializing.orElse(active)
-      
+
       def initializing: Receive = {
         case "FIRST" =>
           recoveryCompleted()
@@ -102,7 +102,7 @@ trait PersistenceDocSpec {
         case other if recoveryFinished =>
           stash()
       }
-      
+
       def recoveryCompleted(): Unit = {
         // perform init after recovery, before any other messages
         // ...