Adam/add defunct states for cart

@dianakhuang 
@wedaly 

This PR introduces two more order statuses—"defunct-cart" and "defunct-paying"—to account for order rows that were sent through the payment processor but not updated in the db. It also adds a "retire" method to order objects, and adds a management command to "retire" lists of orders that should be made defunct.

https://openedx.atlassian.net/browse/ECOM-703
It might be slightly cleaner to use the `ORDER_STATUS_MAP` keys instead of hardcoding this.  Something like:
```
acceptable_keys = set(ORDER_STATUS_MAP.keys())
```