geomirror: leverage LUA records for failover+GeoIP

PowerDNS provides a neat way to implement GeoIP-based redirection and
automatic failover. With GeoLite2-City database, it is able to select
the closest mirror from a list of IPs we provide. Every 60 seconds it
also checks if the mirror's HTTPS URL is working as expected; if that
check fails, it stops giving it out (this acts as automatic failover).