potential bug: Some IPv6Exits do not add the ipv6-policy line to their descriptor

An exit relay is expected to add a ipv6-policy line to its descriptor when:

{{{ IPv6Exit 1 }} and its exit policy allows at least a single IPv6 destination.

There are 10 known cases where the exit didn't generate an ipv6-policy line.

#21355 (moved) should help with debugging.

references: list of exit relays with IPv6 ORPort and no ipv6-policy (if IPv6Exit setting is known, it is shown at the end of the line) https://gist.githubusercontent.com/nusenu/1534d210049fcb04919ae5a4529ea894/raw/4a5611aedb81c5bc01630433c85e8fc818c01a1d/IPv6Exit%25201%253F

https://lists.torproject.org/pipermail/tor-dev/2017-January/011860.html https://lists.torproject.org/pipermail/tor-relays/2017-January/011806.html