Not all the tiles are actually downloaded upon restart

[acerioni]

The bug showed up when running the generate_tilesets.py as of commit 72a4825. It can be reproduced by interrupting the script (CTRL+C) while tiles are being downloaded. Upon restart, the progress bar related to the download task reaches immediately the 100% even if not all the tiles have actually been download. Indeed, the code fails soon after, when checking whether all the expected tiles can be found on the local disk.

I tried investigating the issue but didn't find any explanation. I've just noticed that it can be solved by using the threading Parallel backend instead of loky.

@GwenaelleSa