Summary:
This is not a generic thread local implementation in the sense that it
only takes pointer. But it does support multiple instances per thread
and lets user plugin function to perform cleanup when thread exits or an
instance gets destroyed.

Test Plan: unit test for now

Reviewers: haobo, igor, sdong, dhruba

Reviewed By: igor

CC: leveldb, kailiu

Differential Revision: https://reviews.facebook.net/D16131