!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-persistence/src/main/scala/akka/persistence/JournalProtocol.scala

@@ -4,6 +4,8 @@
 
 package akka.persistence
 
+import scala.collection.immutable
+
 import akka.actor._
 
 /**
@@ -18,6 +20,14 @@ private[persistence] object JournalProtocol {
    */
   case class Delete(persistent: Persistent)
 
+  /**
+   * Instructs a journal to persist a sequence of messages.
+   *
+   * @param persistentBatch batch of messages to be persisted.
+   * @param processor requesting processor.
+   */
+  case class WriteBatch(persistentBatch: immutable.Seq[PersistentImpl], processor: ActorRef)
+
   /**
    * Instructs a journal to persist a message.
    *