Auto Export of download stats

[rgaudin]

We would really benefit from being able to automatically export part of the download stats.

For instance, knowing which ZIM files are currently popular (are the most downloaded) can be used by mirrors to selectively provide a subset of the whole catalog ; could be placed into a faster cache, could be pre-positioned for hotspot image creation, etc.

This is just about download numbers, not about popularity which would be very wrong to consider the same.