Add ComparatorDBTest to test non-default comparators

Summary:
Add some helper functions to make sure DB works well for non-default comparators.
Add a test for SimpleSuffixReverseComparator.

Test Plan: Run the new test

Reviewers: ljin, rven, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D27831