added settle utilities for async library

Hello everybody,

This PR is an extract from my library `future_goodies`, when I started it I didn't saw about Quiver before, and I think it's better for everybody to have those core additions in same place.

I plan to port the rest of future goodies and eventually drop the support there in favor of contributing here if you guys like what I have :)

This is the code of by library: https://github.com/wilkerlucio/future_goodies

Please let me know if you guys like me to port the rest of the library (actually `pipeline` and `sequence` will be dropped since you guys already have similar functionality, maybe others too).