Task::start with a callback
Task::start with a callback

Summary: This is useful to avoid extra allocations when integrating with code that doesn't use folly::futures.

Reviewed By: yfeldblum

Differential Revision: D13812866

fbshipit-source-id: aa76b8cda43d1a781e058f6edbe2ecb8100268de

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