Add an "UnreachableIPv6" flag to relays in consensus health
Split off legacy/trac#24287 (moved).
If you ever have access to relay descriptors in consensus-health, a feature like OnionOO's legacy/trac#21637 (moved) (which will show unreachable addresses on RelaySearch/Atlas) would be great:
If a relay declares an IPv6 address in its descriptor (in an "a" line), but an authority doesn't vote for it, that relay should get the UnreachableIPv6 pseudo-flag from that authority.
If an authority's vote contains any "a" lines with IPv6 addresses, the authority should be marked as knowing the "ReachableIPv6" and "UnreachableIPv6" pseudo-flags.