MaintenanceTool should download with multi-get

MaintenanceTool is downloading archives one after another. This is clearly visible by the download rate taking a dip each time a new archive starts downloading and a new TCP connection needs to find the optimal transmission speed.

Since all packages are known ahead of time, the tools should use HTTP pipelining (or the equivalent mechanism for HTTP/2 or /3, as needed) to avoid the per-connection overhead.