Summary:
Builder and CompactionJob share a lot of fairly complex code. This patch
refactors this code into a separate class, the CompactionIterator. Because the
shared code is fairly complex, this patch hopefully improves maintainability.
While there are is a lot of potential for further improvements, the patch is
intentionally pretty close to the original structure because the change is
already complex enough.

Test Plan: make clean all check && ./db_stress

Reviewers: rven, anthony, yhchiang, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46197