WIP +per #15227 EventsourcedProcessor#persistAsync

**Not for merge - will be retargeted at release-2.3**

Work In Progress / Request For Comments

Wanted to confirm the semantics and get initial feedback with this PR. It still needs more cleaning up and I am hoping to get rid of the mutable.Map in favour of just a List, but need to read into more of the semantics for the WriteJournal still (the resequencer there seems to guarantee proper ordered delivery).

// In this PR I'm still sitting "in the processor", not trying to remove it at the same time as adding persistAsync.