add assert to db Put in db_stress test

Summary:
1. assert db->Put to be true in db_stress
2. begin column family with name "1".

Test Plan: 1. ./db_stress

Reviewers: ljin, yhchiang, dhruba, sdong, igor

Reviewed By: sdong, igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22659