Use thread-local in RequestContext::getStaticContext
Use thread-local in RequestContext::getStaticContext

Summary:
[Folly] Use thread-local in `RequestContext::getStaticContext`.

`folly::SingletonThreadLocal` uses `folly::ThreadLocal`. However, `static FOLLY_TLS` (`static __thread`) is always faster than `folly::ThreadLocal` for thread-local singletons for which iteration is not required.

Reviewed By: djwatson

Differential Revision: D6725091

fbshipit-source-id: 9979f39677284b1051cb109b461097495d77ca17

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