Fix DeleteFile() + enable deleting files oldest files in level 0

Summary:
DeleteFile() call was broken for non-default column family. This fixes it. We might need this feature for mongo.

I also introduced a possibility of deleting oldest file in level 0.

Test Plan: added unit test to deletefile_test

Reviewers: ljin, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24909