extend future api with Future<T>::poll

extend future api with Future<T>::poll

Summary:
Added folly::Future<T>::poll() function with the following semantics:
poll() checks if the promise has been fulfilled, if so it will extract the value of the resulting Try<T> to construct a new Try<T> that is wrapped in an Optional<Try<T>>. If the promise has not been fulfilled an empty Optional is returned.

Test Plan: Added two tests to SugarTest.cpp: one where the promise has been fulfilled and one where it hasn't.

Reviewed By: hannesr@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, arturg, yfeldblum

FB internal diff: D1909480

Tasks: 6166875

Signature: t1:1909480:1426280247:6b239c0f1a15c25ce7664afb4bfb3899993c5775