Use function ptr instead of std::function
Use function ptr instead of std::function

Summary:
Use function ptr instead of std::function

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum, kevin-vigor

Differential Revision: D19277936

fbshipit-source-id: 59c2c79ed07690ffe69461e6a90dbf8b297e51fa

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly