+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-persistence/src/main/scala/akka/persistence/JournalProtocol.scala

@@ -8,8 +8,6 @@ import scala.collection.immutable
 
 import akka.actor._
 
-import akka.persistence.serialization.Message
-
 /**
  * INTERNAL API.
  *
@@ -60,7 +58,7 @@ private[persistence] object JournalProtocol {
    * @param messages messages to be written.
    * @param processor write requestor.
    */
-  final case class WriteMessages(messages: immutable.Seq[PersistentRepr], processor: ActorRef)
+  final case class WriteMessages(messages: immutable.Seq[Resequenceable], processor: ActorRef)
 
   /**
    * Reply message to a successful [[WriteMessages]] request. This reply is sent to the requestor