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Support for column families in TTL DB

Summary:
This will enable people using TTL DB to do so with multiple column families. They can also specify different TTLs for each one.

TODO: Implement CreateColumnFamily() in TTL world.

Test Plan: Added a very simple sanity test.

Reviewers: dhruba, haobo, ljin, sdong, yhchiang

Reviewed By: haobo

CC: leveldb, alberts

Differential Revision: https://reviews.facebook.net/D17859
Merge branch 'master' of https://github.com/facebook/rocksdb