BackgroundFetch will provide a download mechanism, but downloaded files won't live in the Downloads folder, they'll live in origin storage. 

* Fetched files that live in origin storage should be attributed to the origin for quota purposes.
* The system should understand that a fetched file may be moved to a different location and shouldn't be double counted during that time.  (Allows origins to download a file of size close to |quota|)
* Batch management should cancel batch requests if the total size of the request every exceeds quota availability.
* Requests with an estimated size > |quota| should be immediately rejected.