fix TSAN report
fix TSAN report

Summary: Replace `SFRLock` with `folly::SharedMutex` since `SFRLock` triggers TSAN data races in certain runs. `folly::SharedMutex` is a similar concept with similar performance and lack of the TSAN concerns.

Reviewed By: alikhtarov

Differential Revision: D16452269

fbshipit-source-id: 6c3f214d1f40277452cd38d95c95414009772f21

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