Use a typedef for thread-local storage keys (pthread_key_t)
Use a typedef for thread-local storage keys (pthread_key_t)

Summary:
This will result in a cleaner diff when porting to Windows,
which uses a different type for thread-local storage.

Reviewed By: passy

Differential Revision: D19688376

fbshipit-source-id: 0a9ba009ce1814e00b10812b6ed20fa532747722

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