Fix folly::ThreadLocal to have unique singleton in dev builds

Fix folly::ThreadLocal to have unique singleton in dev builds

Summary:This re-uses StaticSingletonManager which was previously used to fix the same issue in folly::Singleton.

Because of the same issue we can no longer use static thread_local for the ThreadEntry. We now rely on pthread_getspecific/pthread_setspecific instead and use static thread_local ThreadEntry* only as a cache (to improve perf).

Reviewed By: yfeldblum

Differential Revision: D2978526

fb-gh-sync-id: cf1d9044afc27b62bd50a1ed931c0c420ae7107e
shipit-source-id: cf1d9044afc27b62bd50a1ed931c0c420ae7107e