Increase MongoEngine reliability for AutoReconnect

Summary:
Should reduce the impact of connection resets from maintenance on
crons and the site by automatically retrying for AutoReconnect
exceptions on read.

Test Plan:
DBShell to prove it works (and handles autoreconnects). Used the site a
bit to make sure there is no strangeness.

Reviewers: josh, jack

Maniphest Tasks: T2699

Differential Revision: http://infra1.corp.contextlogic.com/D9720