When `dns_seems_to_be_broken_for_ipv6()`, exits should stop advertising an IPv6 exit policy. Here's a rough design: * when `dns_seems_to_be_broken_for_ipv6()` is first set to 1, mark the relay descriptor dirty * when rebuilding the descriptor, check `dns_seems_to_be_broken_for_ipv6()` before including an IPv6 exit policy * reset `dns_seems_to_be_broken_for_ipv6()` periodically, maybe every 1-3 days?