+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
Great catch. The wording "atomically" is too strong for out use cases. This atomicity also puts some strain on the journal plugin implementations in `asyncWriteMessages`. I think that we should rethink it, but not for this PR.