DB Sanity Test

Summary:
@kailiu mentioned on meeting yesterday that we sometimes have trouble opening DB created by old version with the new version. This will be very important to test for column families, since I'm changing disk format for the MANIFEST.

I added a tool that can help us test that. Usage:
./db_sanity_test <path> create
will create a bunch of DBs under <path>
<change RocksDB version>
./db_sanity_test <path> verify
will verify consistency of DBs created under <path>

Test Plan: ran the db_sanity_test

Reviewers: kailiu, dhruba, haobo

Reviewed By: kailiu

CC: leveldb, kailiu, xjin

Differential Revision: https://reviews.facebook.net/D16605