comparator_db_test to cover more irregular comparators

Summary:
comparator_db_test now adds verification for three more comparators:
(1) one that store double as string
(2) one that cast uint64 to string
(3) one that concatenate two strings, prefixing their sizes.
(4) one that order by hash of the string

Test Plan:
Run ./comparator_db_test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D27927