Investigate very slow downloads from worker to Kiwix

[rgaudin]

Worker downloads at very slow speed (up to 50Mbps only) while runnig Ookla's speedtest at the same time shows that server can handle 700Mbps (at this time) and client (in docker) can handle close to 1Gbps.

Progress reporting might be an issue

[rgaudin]

This was due to a combination of factors:

• updating the progressbar every time we received data. I'm not sure about the actual impact but we're now updating it at most once every second (unless a download completes).
• download buffer was set to 1KiB which is way too low for large downloads.
• worker was not setting concurrency, resulting in parallel downloads (up to 28 on this worker) which is not efficient on this downloader.

The last two are results of the naive, initial downloader implementation. Although it's working, it's not reliable enough for production so I am adding #9 to the project for rollout.

With the fix, we're seeing reasonable download speeds. A 32GB image (downloading 11GiB) starting on an empty cache takes 5'30 from start to finish.