A PersistentView works the same way as View did previously, except:
* it requires an `peristenceId` (no default is provided)
* messages given to `receive` are NOT wrapped in Persistent()
akka-streams not touched, will update them afterwards on different branch
Also solves #15436 by making persistentId in PersistentView abstract.
(cherry picked from commit dcafaf788236fe6d018388dd55d5bf9650ded696)
Conflicts:
akka-docs/rst/java/lambda-persistence.rst
akka-docs/rst/java/persistence.rst
akka-docs/rst/scala/persistence.rst
akka-persistence/src/main/scala/akka/persistence/Persistent.scala
akka-persistence/src/main/scala/akka/persistence/View.scala
3fd240384c