Synchronize coupled caches in folly::threadlocal_detail::StaticMeta

Synchronize coupled caches in folly::threadlocal_detail::StaticMeta

Summary:
[Folly] Synchronize coupled caches in `folly::threadlocal_detail::StaticMeta`.

The caches should be set together, and only together, because they are coupled. This prevents bugs where one function that sets one cache but not the other cache is inlined into the caller in one module, and another function that reads both caches is inlined into the caller in another module.

Reviewed By: djwatson

Differential Revision: D6435175

fbshipit-source-id: 846c4972b40e525f2c04da6e6609c2ad54f674c0