+per #15424 Added PersistentView, deprecated View

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()

https://github.com/akka/akka/issues/15424
Hm... yeah, the same rationale applies here I guess.
I'll do so + extend the docs on the method that it's mostly about having a different key for snapshots. Is there another use of this I forgot?