[CF] db_stress for column families

Summary:
I had this diff for a while to test column families implementation. Last night, I ran it sucessfully for 10 hours with the command:

   time ./db_stress --threads=30 --ops_per_thread=200000000 --max_key=5000 --column_families=20 --clear_column_family_one_in=3000000 --verify_before_write=1  --reopen=50 --max_background_compactions=10 --max_background_flushes=10 --db=/tmp/db_stress

It is ready to be committed :)

Test Plan: Ran it for 10 hours

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16797