!per #3729 Journaling protocol optimization

- internal batching of individually received Persistent messages
- testing fault tolerance of Processor in presence of random
  * journaling failures
  * processing failures

akka-docs/rst/java/code/docs/persistence/PersistencePluginDocTest.java

@@ -45,12 +45,7 @@ public class PersistencePluginDocTest {
         }
 
         @Override
-        public Future<Void> doWriteAsync(PersistentRepr persistent) {
-            return null;
-        }
-
-        @Override
-        public Future<Void> doWriteBatchAsync(Iterable<PersistentRepr> persistentBatch) {
+        public Future<Void> doWriteAsync(Iterable<PersistentRepr> persistentBatch) {
             return null;
         }