Adding Column Family support in db_bench.
Adding Column Family support in db_bench.

Summary:
Adding num_column_families flag. Adding support for column families in DoWrite and ReadRandom methods.
[Igor, please let me know if this approach sounds good. I shall add it to other methods too.]

Test Plan: Ran fillseq on 1M keys and 10 Column families and ran readrandom.

Reviewers: sdong, yhchiang, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21387

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