When we delete a record on disk, we notify any active migrations by using an "aboutToDelete" callback, which calls "aboutToDeleteForSharding" and gives the DiskLoc which will be affected. However, if multiple migrations are active, it's not clear in which namespace the DiskLoc exists - NamespaceDetails is given, but this is the "value" side of the namespace map, the namespace name itself is not included, just the database.

Since we have the namespace name at the time of aboutToDelete, we should just pass this the the sharding code to make multiple migrations simpler.