+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
You're right that this shouldn't be public for sure. In an effort to minimize the damage for now, I removed this class, and smuggle the non persistent messages inside `PersistentBatch`. 

No one but us should but non-persistent messages in there. The Java API remains unchanged - the `create()` takes `Persistent`'s.

The docs wording is taken from `PersistentBatch`, actually. There it's also "over the top" I think... What it actually does is "deliver these messages, together to the Journal plugin". What the comment meant by atomicity is actually just "did the plugin fail" - `asyncWriteMessages(preparePersistentBatch(resequenceables)) onComplete {`, which does not guarantee anything about "all or none have been persisted.

Anyhow - yes, we need to soften up the words here a bit - I'll open a ticket