+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 #15299 
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
So flushing and looping will make `defer` change the performance characteristics of the actor. If you only do `persist`, `persist` the will be sent as one message to the journal (in the same batch), but `persist`, `defer`, `persist` will be three messages.

That's why I proposed to change  what the batch mechanism in the journal (WriteMessages) accepts.