remove redundant fences in LockFreeRingBuffer
remove redundant fences in LockFreeRingBuffer

Summary: The TurnSequencer adds proper store-release/load-acquire sequencing.

Reviewed By: chadaustin

Differential Revision: D34839255

fbshipit-source-id: f2c4d6d60f89cbe11cfcf147058ec42372d9c5b5

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