* A few more adjustments, found when verifying source compatibility of a few
journal plugins
* Note that PersistentId will be removed with deleteMessage and we should
not break plugins because of that
* Add missing section of at-least-once delivery in migration guide
(cherry picked from commit 6727eac6d07280d277968e2e25db44e02be3b102)
Conflicts:
akka-persistence/src/main/scala/akka/persistence/JournalProtocol.scala
akka-persistence/src/main/scala/akka/persistence/Persistent.scala
akka-persistence/src/main/scala/akka/persistence/Snapshot.scala
akka-persistence/src/main/scala/akka/persistence/journal/AsyncWriteProxy.scala