Summary:
Per title
I found that it was easier to bypass setContext (and not extend RequestContextScopeGuard) to achieve this result.

setShallowCopyContext now directly set the copy as current
the new unsetShallowCopyContext exclusively calls set/onset for the context data that was overriden. This assumes that cost will be fine because the callbackData_ sets are small or empty.
Similar reason they were split from requestData_ in the first place, for RequestContextScopeGuard efficiency.

(Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D8943668

fbshipit-source-id: ad99177429de6b7b65bf95fa0e94334d73c3750d