Sort files correctly in Builder::SaveTo

Summary:
Previously, we used to sort all files by BySmallestFirst comparator and then re-sort level0 files in the Finalize() (recently moved to end of SaveTo).

In this diff, I chose the correct comparator at the beginning and sort the files correctly in Builder::SaveTo.

I also added a verification that all files are sorted correctly in CheckConsistency()

NOTE: This diff depends on D17037

Test Plan: make check. Will also run db_stress

Reviewers: dhruba, haobo, sdong, ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17049