+per #15229 defer for PersistentActor

* Deferred events are not persisted, thus will not participate in replays
  etc. If users want events to be persisted, they can simply use
  `persistAsync` instead.
* This, 3rd, rewrite extends the Persistent hierarchy by a top level
  trait "Resequenceable", which is used to mark every event to be sent
  in sequence back to the PersistentActor. These are split into
  NonPersistentRepr or PersistentRepr, and acted upon accordingly.
* defer is guaranteed to be called, even after persistence failures
* Includes docs updates for java/scala/java8

Resolves #15229
Depends on #15227

Conflicts:
	akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala
	akka-persistence/src/main/scala/akka/persistence/JournalProtocol.scala
	akka-persistence/src/main/scala/akka/persistence/Processor.scala
	akka-persistence/src/test/scala/akka/persistence/PersistentActorSpec.scala
	project/AkkaBuild.scala

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

@@ -319,9 +319,9 @@ public class PersistenceDocTest {
             final ActorRef processor = system.actorOf(Props.create(MyProcessor.class));
 
             public void batchWrite() {
-                processor.tell(PersistentBatch.create(asList(
-                        Persistent.create("a"),
-                        Persistent.create("b"))), null);
+              processor.tell(PersistentBatch.create(asList(
+                  Persistent.create("a"),
+                  Persistent.create("b"))), null);
             }
 
             // ...
@@ -423,7 +423,7 @@ public class PersistenceDocTest {
 
         public void usage() {
             final ActorSystem system = ActorSystem.create("example");
-            //#view-update
+            //#persist-async-usage
             final ActorRef processor = system.actorOf(Props.create(MyPersistentActor.class));
             processor.tell("a", null);
             processor.tell("b", null);
@@ -435,11 +435,56 @@ public class PersistenceDocTest {
             // evt-a-2
             // evt-b-1
             // evt-b-2
-            //#view-update
+            //#persist-async-usage
         }
     };
 
     static Object o10 = new Object() {
+        //#defer
+        class MyPersistentActor extends UntypedPersistentActor {
+
+            @Override
+            public void onReceiveRecover(Object msg) {
+                // handle recovery here
+            }
+
+            @Override
+            public void onReceiveCommand(Object msg) {
+                final Procedure<String> replyToSender = new Procedure<String>() {
+                    @Override
+                    public void apply(String event) throws Exception {
+                        sender().tell(event, self());
+                    }
+                };
+
+                persistAsync(String.format("evt-%s-1", msg), replyToSender);
+                persistAsync(String.format("evt-%s-2", msg), replyToSender);
+                defer(String.format("evt-%s-3", msg), replyToSender);
+            }
+        }
+        //#defer
+
+        public void usage() {
+            final ActorSystem system = ActorSystem.create("example");
+            //#defer-caller
+            final ActorRef processor = system.actorOf(Props.create(MyPersistentActor.class));
+            processor.tell("a", null);
+            processor.tell("b", null);
+
+            // order of received messages:
+            // a
+            // b
+            // evt-a-1
+            // evt-a-2
+            // evt-a-3
+            // evt-b-1
+            // evt-b-2
+            // evt-b-3
+            //#defer-caller
+        }
+    };
+
+    static Object o11 = new Object() {
         //#view
         class MyView extends UntypedView {
             @Override