Summary:
futures::sleep returning a Future leads to continuations easily being run on
the Timekeeper's callback. The goal is to change the return type so that
futures::sleep returns a folly::SemiFuture.

Reviewed By: yfeldblum

Differential Revision: D14069549

fbshipit-source-id: b98e2c76ccbf0a80aed28f90a1f3c63f15eb2e5c