!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/PersistenceDocTest.java

@@ -9,6 +9,8 @@ import scala.Option;
 import akka.actor.*;
 import akka.persistence.*;
 
+import static java.util.Arrays.asList;
+
 public class PersistenceDocTest {
 
     public interface ProcessorMethods {
@@ -237,4 +239,31 @@ public class PersistenceDocTest {
             }
         }
     };
+
+    static Object o6 = new Object() {
+        //#batch-write
+        class MyProcessor extends UntypedProcessor {
+            public void onReceive(Object message) throws Exception {
+                if (message instanceof Persistent) {
+                    Persistent p = (Persistent)message;
+                    if (p.payload().equals("a")) { /* ... */ }
+                    if (p.payload().equals("b")) { /* ... */ }
+                }
+            }
+        }
+
+        class Example {
+            final ActorSystem system = ActorSystem.create("example");
+            final ActorRef processor = system.actorOf(Props.create(MyProcessor.class));
+
+            public void batchWrite() {
+                processor.tell(PersistentBatch.create(asList(
+                        Persistent.create("a"),
+                        Persistent.create("b"))), null);
+            }
+
+            // ...
+        }
+        //#batch-write
+    };
 }

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;