Add warnings when overriding RequestContext

Add warnings when overriding RequestContext

Summary: We currently store the previous value on creation of RequestContextScopeGuard and reset it to the previous one when it goes out of scope - this means that if it's not unset properly somewhere, we'll prolong its life by saving it and restoring later. We need to do this because of some edge cases but a RequestContext should generally never be overridden by a different RequestContext - let's add a warning if this happens

Reviewed By: djwatson, palmtenor

Differential Revision: D3971904

fbshipit-source-id: e72b5f01102d18977d8aa5f8dca5a7802545098a