Move startDurationUs_ to improve cache locality
Move startDurationUs_ to improve cache locality

Summary: As above

Reviewed By: margolei

Differential Revision: D30293702

fbshipit-source-id: 9b8a7a51a5d169cd1309c5179c926b131c43c4a0

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter