Make `next` relaxed
Make `next` relaxed

Summary: We had a rare data race on next this diff is fixing that.

Reviewed By: yfeldblum

Differential Revision: D15120382

fbshipit-source-id: d3f0e521b941fdfcd1d968a42d7e8d240c3f74e6

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly