Implement -[FBTask startAsynchronouslyWithTerminationQueue:] in terms of Futures
Implement -[FBTask startAsynchronouslyWithTerminationQueue:] in terms of Futures

Summary: The same thing can be achieved by mounting a callback on the future. This means that the complexity of managing the callbacks can be completely removed.

Reviewed By: dtorres

Differential Revision: D6496037

fbshipit-source-id: 95f1b56d06885c818256592cf03cdafb614c4038

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl