Better fix than D14707

Summary:
prevent to_mongo() from dereferencing a ReferenceField if it's not
dereferenced, the return value is still the same, just no more implicit
dereferencing

This fixes the problem of first greenlet_swtich happening before user is
is set up by removing the implicit async db query, thus it also fixes
the first handler locale problem

Blame:

Test Plan:
1.made sweeper use this version of mongoengine
2.made my locale spanish
3.logged out, logged in, page is in spanish
4.added prints and pdb and made sure that the implicit dereferencing query didn't occur
5.browswer around the site, did various actions, nothing is broken

Reviewers: josh, adam

Reviewed By: josh

Maniphest Tasks: T9931

Differential Revision: http://phab.wish.com/D14735