!per #3707 Channel enhancements

- Persistent channel
- ConfirmablePersistent message type delivered by channel
- Sender resolution performance improvements
   * unstash() instead of unstashAll()

These enhancements required the following changes

- Unified implementation of processor stash and user stash
- Persistence message plugin API separated from implementation
- Physical deletion of messages

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

@@ -117,7 +117,7 @@ trait PersistenceDocSpec {
 
     class MyDestination extends Actor {
       def receive = {
-        case p @ Persistent(payload, _) ⇒ {
+        case p @ ConfirmablePersistent(payload, _) ⇒ {
           println(s"received ${payload}")
           p.confirm()
         }
@@ -243,4 +243,17 @@ trait PersistenceDocSpec {
     //#batch-write
     system.shutdown()
   }
+
+  new AnyRef {
+    import akka.actor._
+    trait MyActor extends Actor {
+      val destination: ActorRef = null
+      //#persistent-channel-example
+      val channel = context.actorOf(PersistentChannel.props(),
+        name = "myPersistentChannel")
+
+      channel ! Deliver(Persistent("example"), destination)
+      //#persistent-channel-example
+    }
+  }
 }