Summary:
This algorithm takes an awaitable and a duration and
cancels the operation and completes with a `FutureTimeout`
error if the operation does not complete within the specified
duration.

This differs from `timed_wait()` in that it still waits
for the child operation to complete, even once the timeout
has elapsed, rather than just detaching from the
operation and letting it complete in the background.

This relies on the operation responding to cancellation
in a timely manner for the algorithm to have the effect
of returning shortly after the timeout elapses.

The benefit of waiting for the child operation to complete,
however, is that it allows you to safely pass parameters by
reference to the child operation and be safe in knowing that
the child operation will complete before the awaiting
coroutine resumes and potentially destroys those parameters.

Reviewed By: yfeldblum

Differential Revision: D22199932

fbshipit-source-id: 4f6295a23a860ec52b639eb56a02f8029ffb9009