db_bench --num_hot_column_families to be default off

Summary: Having --num_hot_column_families default on fails some existing regression tests. By default turn it off

Test Plan: Run db_bench to make sure it is default off.

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D30705