Future<T>::onTimeout(Duration, function<T()>, Timekeeper*=nullptr)

Future<T>::onTimeout(Duration, function<T()>, Timekeeper*=nullptr)

Summary:
(or func returns Future<T>)

Invoke and respond to a timeout with a callback, rather than using `within` and adding `onError` or `then` or something.

Test Plan: new tests

Reviewed By: davejwatson@fb.com

Subscribers: jsedgwick, yfeldblum, trunkagent, fugalh, exa, folly-diffs@

FB internal diff: D1763174

Tasks: 4548494

Signature: t1:1763174:1423074062:05cec1dfb1110b31b599033949ebe0ee70dd0552