db_bench to add an option as number of hot column families to add to

Summary:
Add option --num_hot_column_families in db_bench. If it is set, write options will first write to that number of column families, and then move on to next set of hot column families. The working set of column families can be smaller than total number of CFs.

It is to test how RocksDB can handle cold column families

Test Plan: Run db_bench with  --num_hot_column_families set and not set.

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30663