Summary:
This diff is more about my question when reading compaction codes,
instead of a normal diff. I don't quite understand the logic here.

Test Plan: I didn't do any test. If this is a bug, I will continue doing some test.

Reviewers: haobo, dhruba, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11661