Detect unusually slow downloads and restart / resume them

[ngld]

Under certain circumstances, at the end of a mod installation, only one download is left but it's for some reason really slow (i.e. 25 KiB/s) which makes the mod installation much slower than it should be. Knossos should detect whenever a download is significantly slower than previous downloads and either try using a better/faster mirror or resume the download since this issue sometimes only affects specific TCP connections which means that reconnecting fixes it.