Use weak_ptr to hold future context in timekeeper to allow clean up when future complete

Use weak_ptr to hold future context in timekeeper to allow clean up when future complete

Summary:
Before this change, future context will not be cleaned up until timekeeper times out. These objects has been occupying memory when more shorter future tasks are registered.
Switch to use weak ptr to hold context, so that context objects are deallocated as soon as the future completes (or times out)

Reviewed By: yfeldblum

Differential Revision: D5692040

fbshipit-source-id: b3b74a29b2ccafef6c4a06011699b069feb3a847