make key evenly distributed between 0 and FLAGS_num

Summary:
The issue is that when FLAGS_num is small, the leading bytes of the key
are padded with 0s. This makes all keys have the same prefix 00000000

Most of the changes are just to make lint happy

Test Plan: ran db_bench

Reviewers: sdong, haobo, igor

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16317