Summary: There's no need to extensively call into steady_clock, because the time is unlikely to change. And if it does change, then it means that we'd do incorrect math (like firing timeout 10ms earlier).

Reviewed By: djwatson, stevegury

Differential Revision: D13541503

fbshipit-source-id: cc46c8a6bd6a72544803a22d16235be54fa94cc4