Improve FutureAwaitable::await_ready
Improve FutureAwaitable::await_ready

Summary: This helps avoid executor re-schedule when Future is ready.

Reviewed By: yfeldblum

Differential Revision: D13805939

fbshipit-source-id: f553c7a581882a7b53b004fd6bbb5087ea5787f8

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly