Prop 311: 4.3.2. Add AssumeReachableIPv6 Option and Consensus Parameter

We add an AssumeReachableIPv6 torrc option and consensus parameter.

If IPv6 ORPort checks have bugs that impact the health of the network, they can be disabled by setting AssumeReachableIPv6=1 in the consensus parameters.

If IPv6 ORPort checks have bugs that impact a particular relay (or bridge), they can be disabled by setting "AssumeReachableIPv6 1" in the relay's torrc.

This option disables IPv6 ORPort reachability checks, so relays publish their descriptors if their IPv4 ORPort reachability checks succeed. (Unlike AssumeReachable, AssumeReachableIPv6 has no effect on the existing dirauth IPv6 reachability checks, which connect directly to relay ORPorts.)

The default for the torrc option is "auto", which checks the consensus parameter. If the consensus parameter is not set, the default is "0".

"AssumeReachable 1" overrides all values of "AssumeReachableIPv6", disabling both IPv4 and IPv6 ORPort reachability checks. Tor should warn if AssumeReachable is 1, but AssumeReachableIPv6 is 0. (On directory authorities, "AssumeReachable 1" also disables dirauth IPv4 and IPv6 reachability checks, which connect directly to relay ORPorts. AssumeReachableIPv6 does not disable directory authority to relay IPv6 checks.)

See proposal 311, section 4.3.2: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt#n403