Clean up RequestContext

Clean up RequestContext

Summary: Using a ThreadLocal cleans up the code quite a bit.  Also reuse the shared_ptr in create instead of creating a new one saves an allocation

Test Plan:
fbconfig thrift/lib/cpp/test:RequestContextTest; fbmake runtests

fbconfig common/services/cpp/test:trace_test; fbmake runtests

Reviewed By: hans@fb.com

Subscribers: trunkagent, doug, alandau, bmatheny, njormrod, mshneer, folly-diffs@, vloh

FB internal diff: D1663960

Signature: t1:1663960:1415390250:36d9b772016d2a12d804e98edbc1725af882e507