Summary:
This diff reduces number of memory allocation in folly::ThreadWheelTimekeeper.after() method for a bit.

 * std::shared_ptr(new T) is replaced with std::make_shared<T>()
 * folly::Promise is stored by value

Reviewed By: yfeldblum

Differential Revision: D6172017

fbshipit-source-id: 41bf123f10570c76d64eaac1800b7e65fe381110