Unfriending classes

Summary:
In this diff I made some effort to reduce usage of friending. To do that, I had to expose Compaction::inputs_ through a method inputs(). Not sure if this is a good idea, there is a trade-off. I think it's less confusing than having lots of friends.

I also thought about other friendship relationships, but they are too much tangled at this point. Once you friend two classes, it's very hard to unfriend them :)

Test Plan: make check

Reviewers: haobo, kailiu, sdong, dhruba

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15267