[CF] DB test to run on non-default column family

Summary:
This is a huge diff and it was hectic, but the idea is actually quite simple. Every operation (Put, Get, etc.) done on default column family in DBTest is now forwarded to non-default ("pikachu"). The good news is that we had zero test failures! Column families look stable so far.

One interesting test that I adapted for column families is MultiThreadedTest. I replaced every Put() with a WriteBatch writing to all column families concurrently. Every Put in the write batch contains unique_id. Instead of Get() I do a multiget across all column families with the same key. If atomicity holds, I expect to see the same unique_id in all column families.

Test Plan: This is a test!

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16149