Health-check: GFD hosts

[ro-tex]

We need to add a new heath-check item, which is good for download contracts. Unfortunately, it's not very easy to come up with a good threshold because the value is dynamic. One idea for a solution is to check the values of all hosts and make sure we're not more than 30% under the average. We can potentially exposed that value on siasky.tools.

Original discussion: https://discord.com/channels/341359001797132308/674433179968602113/877940460792983602