Put table factory back to public api

Summary:
Previous I am too ambitious to hide every detail about table factory
to internal api. However, we cannot pass the compilatoin for external
users since we use table factory as the shared_ptr, which requires
the definition of table factory's destructor.

Test Plan: make check;

Reviewers: sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15861