+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
+1 Bjorn, I was actually on that path and strayed from it when pointed to "just loop". I agree that degrading perf is not a tradeoff we have to do here - 3rd impl should be PRed soon then :-)