Make find_iter cursors eventually time out

Summary:
10minutes by default
not configurable, only off or 10minutes-
https://jira.mongodb.org/browse/SERVER-8188

Blame:

Test Plan: run a find_iter with local mongoengine symlinkied

Reviewers: adam, jack

Reviewed By: adam

Maniphest Tasks: T13594

Differential Revision: http://phab.wish.com/D19368