Summary: [Folly] Avoid `std::forward` in `folly/futures/` since function template instantiation is expensive at compile time and `std::forward` is, when used correctly, nothing more than library sugar for straightforward syntax.

Reviewed By: LeeHowes

Differential Revision: D22673234

fbshipit-source-id: f88bc8c8aa13797ae6eccf1a421a47ff135589e1