[Feature Request] Prefer nameservers that respond over those that don't

[phillip-stephens]

Of the available nameservers for a domain, ZDNS will round-robin randomly thru this set. While we cache responses, if a nameserver never replies then ZDNS will still try to connect.

A potential performance gain could be we cache nameservers that don't respond or we cache their failure rates and use that to prioritize selecting the nameservers that respond more.

This "negative cache" could help ZDNS avoid attempting to connect to a nameserver we know is unreachable.