When an exit tells a client about an IPv6-only hostname, the client should choose another IPv6 exit

When legacy/trac#21311 (moved) and legacy/trac#21310 (moved) are finished, we need to make clients choose an IPv6 exit when an exit tells them they are connecting to an IPv6-only site

We can't choose an exit for the specific IPv6 address, because that leads to tagging attacks.

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