RequestContext: replace F14FastMap by std::unordred_map
RequestContext: replace F14FastMap by std::unordred_map

Summary: Production data shows that it performs significantly worse under common RequestContext load (few elements, clang lto compiled).

Reviewed By: djwatson

Differential Revision: D9182929

fbshipit-source-id: 0c638268d9b6cd2451b48ccc4e228faf90f0e18e

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