Try to strip ETags from downloads in DAVE

If we resume from another mirror (which can only happen if the initially picked mirror has been removed from the pool between the time when the download was paused, and the time it’s resumed; since by default we resume using the same mirror): we do that by patching the download object to change the target URL, and then it might be that previous ETag headers cached for this download could easily be dropped by DAVE; this might be as simple as adding one single line similar to the one that modifies the URL.

If we manage to do this, we could drop the requirement for mirror operators to disable ETags on their servers.

Parent Task: #12328 (closed)

Original created by @u on 11797 (Redmine)