!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-lambda/src/main/java/sample/persistence/PersistentActorExample.java

@@ -4,13 +4,13 @@
 
 package sample.persistence;
 
-//#eventsourced-example
+//#persistent-actor-example
 
 import akka.actor.ActorRef;
 import akka.actor.ActorSystem;
 import akka.actor.Props;
 import akka.japi.pf.ReceiveBuilder;
-import akka.persistence.AbstractEventsourcedProcessor;
+import akka.persistence.AbstractPersistentActor;
 import akka.persistence.SnapshotOffer;
 import scala.PartialFunction;
 import scala.runtime.BoxedUnit;
@@ -73,7 +73,7 @@ class ExampleState implements Serializable {
     }
 }
 
-class ExampleProcessor extends AbstractEventsourcedProcessor {
+class ExampleProcessor extends AbstractPersistentActor {
     private ExampleState state = new ExampleState();
 
     public int getNumEvents() {
@@ -104,9 +104,9 @@ class ExampleProcessor extends AbstractEventsourcedProcessor {
             match(String.class, s -> s.equals("print"), s -> System.out.println(state)).build();
     }
 }
-//#eventsourced-example
+//#persistent-actor-example
 
-public class EventsourcedExample {
+public class PersistentActorExample {
     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-java8");

akka-samples/akka-sample-persistence-java-lambda/src/main/java/sample/persistence/ProcessorFailureExample.java

@@ -11,8 +11,6 @@ import akka.japi.pf.ReceiveBuilder;
 import akka.persistence.AbstractProcessor;
 import akka.persistence.Persistent;
 import scala.Option;
-import scala.PartialFunction;
-import scala.runtime.BoxedUnit;
 
 import java.util.ArrayList;
 

akka-samples/akka-sample-persistence-java-lambda/tutorial/index.html

@@ -72,7 +72,7 @@ current processor state to <code>stdout</code>.
 
 <p>
 To run this example, go to the <a href="#run" class="shortcut">Run</a> tab, and run the application main class
-<b><code>sample.persistence.EventsourcedExample</code></b> several times.
+<b><code>sample.persistence.PersistentActorExample</code></b> several times.
 </p>
 </div>