!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-scala/src/main/scala/sample/persistence/PersistentActorExample.scala

@@ -1,6 +1,6 @@
 package sample.persistence
 
-//#eventsourced-example
+//#persistent-actor-example
 import akka.actor._
 import akka.persistence._
 
@@ -13,7 +13,7 @@ final case class ExampleState(events: List[String] = Nil) {
   override def toString: String = events.reverse.toString
 }
 
-class ExampleProcessor extends EventsourcedProcessor {
+class ExampleProcessor extends PersistentActor {
   var state = ExampleState()
 
   def updateState(event: Evt): Unit =
@@ -39,9 +39,9 @@ class ExampleProcessor extends EventsourcedProcessor {
   }
 
 }
-//#eventsourced-example
+//#persistent-actor-example
 
-object EventsourcedExample extends App {
+object PersistentActorExample extends App {
 
   val system = ActorSystem("example")
   val processor = system.actorOf(Props[ExampleProcessor], "processor-4-scala")

akka-samples/akka-sample-persistence-scala/tutorial/index.html

@@ -90,7 +90,7 @@ 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>