Fix laggy fastdl

[JustinBenedick]

For some reason maps are slow to download. For example star_trek_rats_v2 is less than 4mb and would take 45 minutes to download. Not sure if its hitting fastdl server or not.

[JustinBenedick]

https://forums.alliedmods.net/showthread.php?t=295422&highlight=sv_downloadurl << this maybe the right solution. sv_download 0 will force to use fastdl