+per defer for PersistentActor

note: defer does not persist anything (does not even hit the journal). I think this is enough, as if someone wants to persist stuff (get replayable stuff), they can just use persist methods.

Refs https://github.com/akka/akka/issues/15230
Depends on #15227

TODO: 
- [x] try to implement using same mechanisms as `persistAsync`, remove the 3rd state, make the Journal not persist `LoopStuff` wrapped messages.
- [x] update the persistence lambda docs
Hm... I was assuming *not* calling defer would be the right thing to do here, but maybe you're right. I'll also document this explicitly - that defer is called after success/failure of persisting. Ordering is guaranteed, but success is not - you can still "handle things" in the `defer` if you previously got a failure I guess.