Log download info in dets: Which files have been dowloaded, and how?

[nroi]

For testing, it would be useful to have a log that stores which files have been downloaded, and whether they have been served via HTTP, via cache, or both (first cache, then HTTP). This would make it possible to test not only that we receive the correct file (sha256 checksums match), but also to test that files are really served from cache when possible.