+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
ok, thanks for asking, I agree that it is a hidden assumption, and it would be nice with a comment and a IllegalState check here.

I hope it is correct because of the demandCount/progressCount: https://github.com/akka/akka/pull/15999/files#diff-6fbc12946c03531e5829cf792bd87355R55