Simplify observable state
Simplify observable state

Summary: Merge two mutexes together and get rid of atomics.

Reviewed By: yfeldblum

Differential Revision: D14236244

fbshipit-source-id: 0b97d519e72322377e225a0cc8975e3a2a86ec31

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