Remove `Deferrable[T]` and replace with POF (plain old Future)

closes #15962
That is something that we should discuss.
It doesn't *have to be* public API, but leaving it public would allow users to write more efficient akka-http marshallers and unmarshallers themselves (among other things), if they care to.
Knowing the spray users I'm sure that there are some who will ask for this.