!per #3681 Performance and consistency improvements

- batch-write of persistent messages (user API)
- batch-write of events (in EventsourcedProcessor)
- command processing in EventsourcedProcessor by unstashing messages one-by-one from the internal stash
   * fixes performance issues that come up with unstashAll
- commands are not looped through journal actor but processed directly
- initial performance tests
  * command sourcing
  * event sourcing
  * event sourcing with user stash operations
- suppress stack traces in tests

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

@@ -225,4 +225,22 @@ trait PersistenceDocSpec {
       maxTimestamp = System.currentTimeMillis))
     //#snapshot-criteria
   }
+
+  new AnyRef {
+    import akka.actor.Props
+    //#batch-write
+    class MyProcessor extends Processor {
+      def receive = {
+        case Persistent("a", _) ⇒ // ...
+        case Persistent("b", _) ⇒ // ...
+      }
+    }
+
+    val system = ActorSystem("example")
+    val processor = system.actorOf(Props[MyProcessor])
+
+    processor ! PersistentBatch(Vector(Persistent("a"), Persistent("b")))
+    //#batch-write
+    system.shutdown()
+  }
 }