Refactor with_timer_lwt
Refactor with_timer_lwt

Summary: This just pulls the meat of the function out into helpers, in preparation for adding a non-Lwt version of this function in the next diff.

Reviewed By: gkz

Differential Revision: D19559116

fbshipit-source-id: 4cd886ffe1e0123bea1d7f79366fc2374cbc25b0

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow