Support for asynchronous transformation

[qmac]

Many APIs (including some higher quality ones) perform jobs with a significant time delay (what I mean by "asynchronous" here). One prime example of this is the GoogleVideoIntelligenceAPIExtractor. Other audio and video-based APIs have this property as well.

It would be good to make some generalized interface for handling APIs that will return jobs asynchronously. Done correctly, this may require significant engineering effort.

Maybe the onus is on the user to use pliers asynchronously, I'm not sure/would need to think more. Open to discussion.