Check a consensus parameter before activating onion service IPv6 features
We've implemented legacy/trac#23577 (moved), but it looks like none of the other onion service IPv6 code will be ready for 0.3.3.
(We want to do the relay IPv6 code first.)
If we merge this in 0.3.3, then services will be able to distinguish 0.3.2 and 0.3.3 (and later) clients, when the rend point is dual-stack.
Do we want to add a torrc option / consensus parameter for v3 onion service IPv6? Or are we happy with this information leak?