lftpsync: limit jobs number
lftpsync: limit jobs number

As mirrors4 has 80 cores, default settings will lead to ridiciously high concurrency and get our IP address disliked.

ustclug/ustcmirror-images,ustclug/ustcmirror-images,ustclug/ustcmirror-images,ustclug/ustcmirror-images,ustclug/ustcmirror-images