!per #3681 Performance and consistency improvements

- batch-write of persistent messages (user API)
- batch-write of events (in EventsourcedProcessor)
- command processing in EventsourcedProcessor by unstashing messages one-by-one from the internal stash
   * fixes performance issues that come up with unstashAll
- commands are not looped through journal actor but processed directly
- initial performance tests
  * command sourcing
  * event sourcing
  * event sourcing with user stash operations
- suppress stack traces in tests

akka-docs/rst/scala/code/docs/persistence/PersistencePluginDocSpec.scala

@@ -6,6 +6,7 @@ package docs.persistence
 
 //#plugin-imports
 import scala.concurrent.Future
+import scala.collection.immutable.Seq
 //#plugin-imports
 
 import com.typesafe.config._
@@ -69,6 +70,7 @@ class PersistencePluginDocSpec extends WordSpec {
 
 class MyJournal extends AsyncWriteJournal {
   def writeAsync(persistent: PersistentImpl): Future[Unit] = ???
+  def writeBatchAsync(persistentBatch: Seq[PersistentImpl]): Future[Unit] = ???
   def deleteAsync(persistent: PersistentImpl): Future[Unit] = ???
   def confirmAsync(processorId: String, sequenceNr: Long, channelId: String): Future[Unit] = ???
   def replayAsync(processorId: String, fromSequenceNr: Long, toSequenceNr: Long)(replayCallback: (PersistentImpl) ⇒ Unit): Future[Long] = ???