  o Major features (IPv6, relay):
    - Relays may extend circuits over IPv6, if the relay has an IPv6 ORPort,
      and the client supplies the other relay's IPv6 ORPort in the EXTEND2
      cell. IPv6 extends will be used by the relay IPv6 ORPort self-tests in
      33222. Closes ticket 33817.
    - Consider IPv6-only EXTEND2 cells valid on relays. Log a protocol warning
      if the IPv4 or IPv6 address is an internal address, and internal
      addresses are not allowed. But continue to use the other address, if it
      is valid. Closes ticket 33817.
    - If a relay can extend over IPv4 and IPv6, it chooses between them
      uniformly at random. Closes ticket 33817.
    - Re-use existing IPv6 connections for circuit extends. Closes ticket 33817.
