Column family logging

Summary:
Now that we have column families involved, we need to add extra context to every log message. They now start with "[column family name] log message"

Also added some logging that I think would be useful, like level summary after every flush (I often needed that when going through the logs).

Test Plan: make check + ran db_bench to confirm I'm happy with log output

Reviewers: dhruba, haobo, ljin, yhchiang, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18303