+str #15996 Add unorderedMapAsync

* rename mapFuture to mapAsync
* add unorderedMapAsync
* extract common things for emits to a trait

I'm not sure about the name `unorderedMapAsync`. Any better suggestions?

Refs #15996
The name feels a bit backwards to me. How about `mapAsyncUnordered`, since it's so close to `mapAsync` in what it does.