Call a function for each element in a queryset (actually, any list).
Features:
* stable memory usage (thanks to Django paginators)
* progress indicators
* wraps batches in transactions
* can take managers or even models (e.g., `Assertion.objects`)
* warns about `DEBUG`.
* handles failures of single items without dying in general.
* stable even if items are added or removed during processing (gets a list of ids at the start)
Returns a `Status` object, with the following interesting attributes
* `total`: number of items in the queryset
* `num_successful`: count of successful items
* `failed_ids`: list of ids of items that failed
- queryset
- progress
- callback
- map
- foreach
- iterator