!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/java/akka/persistence/journal/japi/SyncWritePlugin.java

@@ -15,6 +15,14 @@ interface SyncWritePlugin {
      */
     void doWrite(PersistentImpl persistent) throws Exception;
 
+    /**
+     * Plugin Java API.
+     *
+     * Synchronously writes a batch of persistent messages to the journal. The batch write
+     * must be atomic i.e. either all persistent messages in the batch are written or none.
+     */
+    void doWriteBatch(Iterable<PersistentImpl> persistentBatch);
+
     /**
      * Plugin Java API.
      *