Use SharedMutex in Observable
Use SharedMutex in Observable

Summary: SharedMutex scales better and will reduce compile times.

Reviewed By: yfeldblum

Differential Revision: D6063061

fbshipit-source-id: 7e6c6dca5aba9891cbeeff2d75e21abdbd3beeeb

facebook/wangle,facebook/wangle,facebook/wangle