This was causing issues on some sites, having the const, because this really is heavily
dependent on upload speed. We request 5-10 URLs at a time with this endpoint; for
a 1.5GB upload with 5mb parts this could mean 60 requests to the server to get all
the part URLs. If the user's upload speed is super fast they may request all 60
batches in a minute, if it is slow they may request 5 batches in a minute.
The other external upload endpoints are not hit as often, so they can stay as constant
values for now. This commit also increases the default to 20 requests/minute.
244836ddd4