Summary:
This enables user to add a TTL column family to normal DB.

Next step should be to expand StackableDB and create StackableColumnFamily, such that users can for example add geo-spatial column families to normal DB.

Test Plan: added a test

Reviewers: dhruba, haobo, ljin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18201