distinct query for mongo

Summary:
objects().distinct() is used quite a bit in dashy.
in some cases, the distinct set may be much smaller than the total set so probly should do this db level
(for instance, get/type-id for auto wish source only has 4 in distinct set)

Test Plan: User.distinct({'name':'David Zhang'}, "gender") gave [1]

Reviewers: adam, jack, josh

Reviewed By: adam

Differential Revision: http://infra1.corp.contextlogic.com/D10240