Implement IPv6 ORPort reachability fallback

Implement the IPv6 ORPort reachability fallback from #24404 (moved).

changed milestone to %Tor: unspecified

added 034-removed-20180328 034-triage-20180328 component::core tor/tor ipv6 milestone::Tor: unspecified parent::24403 points::1 priority::medium severity::normal status::new tor-relay type::enhancement labels

Deferring various "new"-status enhancement tickets to 0.3.4

Trac:
Milestone: Tor: 0.3.3.x-final to Tor: 0.3.4.x-final

The 0.3.3 freeze deadline has passed, all these children of #24403 (moved) belong in 0.3.4

Trac:
Keywords: N/A deleted, 034-triage-20180328 added

Per our triage process, these tickets are pending removal from 0.3.4.

Trac:
Keywords: N/A deleted, 034-removed-20180328 added

These tickets, tagged with 034-removed-*, are no longer in-scope for 0.3.4. We can reconsider any of them, if time permits.

Trac:
Milestone: Tor: 0.3.4.x-final to Tor: unspecified

Removing sponsor V as we do not have more time to include this tickets in the sponsor.

Removing sponsor from tickets that we do not have time to fit in the remain of this sponsorship.

Trac:
Sponsor: SponsorV-can to N/A

changed time estimate to 8h

moved to tpo/core/tor#24406 (closed)