IPv6-only Tor2web should use a 3-hop path on unreachable or failed intro or rend

Just like single onion services, tor2web should use a 3-hop path if the intro or rend point is unreachable (using ReachableAddresses), or retry a 3-hop path if the connection fails.

This is more robust than placing requirements on hidden service intro point selection.

Was: IPv6-only Tor2web: Hidden services should choose an IPv6-capable intro point ~~This enables IPv6-only Tor2web instances, at the cost of putting slightly more load on dual-stack relays.

(If 25% of relays are dual-stack, the chance that no relays out of 6 will be dual-stack is about 10%. So this is only a small increase in load.)~~