!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/journal/japi/SyncWriteJournal.scala

@@ -4,6 +4,9 @@
 
 package akka.persistence.journal.japi
 
+import scala.collection.immutable
+import scala.collection.JavaConverters._
+
 import akka.persistence.journal.{ SyncWriteJournal ⇒ SSyncWriteJournal }
 import akka.persistence.PersistentImpl
 
@@ -16,6 +19,9 @@ abstract class SyncWriteJournal extends AsyncReplay with SSyncWriteJournal with
   final def write(persistent: PersistentImpl) =
     doWrite(persistent)
 
+  final def writeBatch(persistentBatch: immutable.Seq[PersistentImpl]) =
+    doWriteBatch(persistentBatch.asJava)
+
   final def delete(persistent: PersistentImpl) =
     doDelete(persistent)