Prop 311: 5. Declare Support for Subprotocol Version "Relay=3"
This ticket depends on relay IPv6 extends in legacy/trac#33220 (moved).
We reserve Tor subprotocol "Relay=3" for tor versions where:
- relays may perform IPv6 extends, and
- bridges might not perform IPv6 extends, as described in this proposal.
See proposal 311, section 5: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt#n601