Add a `--slow` option for `sync` command

On very low connections (< 100 Kb/s), the sync will likely fail at some point.

But it could work if the number of download slots was limited to 1. I've tested it, it works.

Also, there is a bug on local storage of downloaded chunks: they are not stored on a brand new installation, so when the download eventually fail, we have to start the whole download again.

Diagnostic: replace localNumber == 0 by localNumber <= 0 in the downloadChunk function.