+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
well, it's possible to use `PersistentActor` with command sourcing
(I think it is enough to say "Use `akka.persistence.PersistentActor` instead.")