!per persistAsync

Breaks binary compatibility because adding new methods to Eventsourced
trait. Since akka-persistence is experimental this is ok, yet
source-level compatibility has been perserved thankfuly :-)

Deprecates:
* Rename of EventsourcedProcessor -> PersistentActor
* Processor -> suggest using PersistentActor
* Migration guide for akka-persistence is separate, as wel'll deprecate in minor versions (its experimental)
* Persistent as well as ConfirmablePersistent - since Processor, their
  main user will be removed soon.

Other changes:
* persistAsync works as expected when mixed with persist
* A counter must be kept for pending stashing invocations
* Uses only 1 shared list buffer for persit / persistAsync
* Includes small benchmark
* Docs also include info about not using Persistent() wrapper
* uses java LinkedList, for best performance of append / head on
  persistInvocations; the get(0) is safe, because these msgs only
  come in response to persistInvocations
* Renamed internal *MessagesSuccess/Failure messages because we kept
  small mistakes seeing the class "with s" and "without s" as the same
* Updated everything that refered to EventsourcedProcessor to
  PersistentActor, including samples

Refs #15227

Conflicts:
	akka-docs/rst/project/migration-guides.rst
	akka-persistence/src/main/scala/akka/persistence/JournalProtocol.scala
	akka-persistence/src/main/scala/akka/persistence/Persistent.scala
	akka-persistence/src/test/scala/akka/persistence/PersistentActorSpec.scala
	project/AkkaBuild.scala

akka-samples/akka-sample-persistence-java/src/main/java/sample/persistence/EventsourcedExample.java

@@ -1,120 +0,0 @@
-package sample.persistence;
-
-//#eventsourced-example
-import java.io.Serializable;
-import java.util.ArrayList;
-
-import akka.actor.*;
-import akka.japi.Procedure;
-import akka.persistence.*;
-
-import static java.util.Arrays.asList;
-
-class Cmd implements Serializable {
-    private final String data;
-
-    public Cmd(String data) {
-        this.data = data;
-    }
-
-    public String getData() {
-        return data;
-    }
-}
-
-class Evt implements Serializable {
-    private final String data;
-
-    public Evt(String data) {
-        this.data = data;
-    }
-
-    public String getData() {
-        return data;
-    }
-}
-
-class ExampleState implements Serializable {
-    private final ArrayList<String> events;
-
-    public ExampleState() {
-        this(new ArrayList<String>());
-    }
-
-    public ExampleState(ArrayList<String> events) {
-        this.events = events;
-    }
-
-    public ExampleState copy() {
-        return new ExampleState(new ArrayList<String>(events));
-    }
-
-    public void update(Evt evt) {
-        events.add(evt.getData());
-    }
-
-    public int size() {
-        return events.size();
-    }
-
-    @Override
-    public String toString() {
-        return events.toString();
-    }
-}
-
-class ExampleProcessor extends UntypedEventsourcedProcessor {
-    private ExampleState state = new ExampleState();
-
-    public int getNumEvents() {
-        return state.size();
-    }
-
-    public void onReceiveRecover(Object msg) {
-        if (msg instanceof Evt) {
-            state.update((Evt) msg);
-        } else if (msg instanceof SnapshotOffer) {
-            state = (ExampleState)((SnapshotOffer)msg).snapshot();
-        }
-    }
-
-    public void onReceiveCommand(Object msg) {
-        if (msg instanceof Cmd) {
-            final String data = ((Cmd)msg).getData();
-            final Evt evt1 = new Evt(data + "-" + getNumEvents());
-            final Evt evt2 = new Evt(data + "-" + (getNumEvents() + 1));
-            persist(asList(evt1, evt2), new Procedure<Evt>() {
-                public void apply(Evt evt) throws Exception {
-                    state.update(evt);
-                    if (evt.equals(evt2)) {
-                        getContext().system().eventStream().publish(evt);
-                    }
-                }
-            });
-        } else if (msg.equals("snap")) {
-            // IMPORTANT: create a copy of snapshot
-            // because ExampleState is mutable !!!
-            saveSnapshot(state.copy());
-        } else if (msg.equals("print")) {
-            System.out.println(state);
-        }
-    }
-}
-//#eventsourced-example
-
-public class EventsourcedExample {
-    public static void main(String... args) throws Exception {
-        final ActorSystem system = ActorSystem.create("example");
-        final ActorRef processor = system.actorOf(Props.create(ExampleProcessor.class), "processor-4-java");
-
-        processor.tell(new Cmd("foo"), null);
-        processor.tell(new Cmd("baz"), null);
-        processor.tell(new Cmd("bar"), null);
-        processor.tell("snap", null);
-        processor.tell(new Cmd("buzz"), null);
-        processor.tell("print", null);
-
-        Thread.sleep(1000);
-        system.shutdown();
-    }
-}