!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/java/code/docs/persistence/PersistencePluginDocTest.java

@@ -45,6 +45,11 @@ public class PersistencePluginDocTest {
             return null;
         }
 
+        @Override
+        public Future<Void> doWriteBatchAsync(Iterable<PersistentImpl> persistentBatch) {
+            return null;
+        }
+
         @Override
         public Future<Void> doDeleteAsync(PersistentImpl persistent) {
             return null;