Summary:
Timekeeper.h had a weird circular dependence issue that would show up when
included from within the future implementation headers.  The stuff in
Timekeeper.h should ideally be within the future headers themselves

Reviewed By: yfeldblum, Orvid

Differential Revision: D7787275

fbshipit-source-id: a6309c0cc5edca4f5bd12d2651c828299681df54