Prop 312: 4. Update Directory Spec for IPv6 X-Your-Address-Is

We should explicitly support IPv6 X-Your-Address-Is HTTP headers in the tor directory protocol.

We need to decide whether IPv6 X-Your-Address-Is headers should be formatted enclosed in square brackets. To help with this decision, we should look at what relays currently do (in tor 0.3.5 to 0.4.2, as of January 2020).

See proposal 312, section 4, for a draft of this change: https://gitweb.torproject.org/torspec.git/tree/proposals/312-relay-auto-ipv6-addr.txt#n1432

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