use ipv6

[warner]

Shawn Willden points out that IPv6 is a likely way to deal with the servers-behind-NAT problem (#169, #445, #49, #50), since a lot of ISPs offer v6 tunnel services these days, which effectively gives the server host a publically-reachable v6 address.

To make this work, we need a couple of things, probably in the following order:

• python must handle v6 addresses, in the socket module
• twisted must handle v6 addresses: connecting to them, reporting them when asking a socket who it is connected to, and enumerating them on listenable interfaces. The existing IPv4Address class must grow a corresponding IPv6Address partner.
• foolscap must handle them: putting them in connection hints (​foolscap#60 has some notes on formats), connecting to them, etc
• tahoe must handle them: formatting them into Welcome page listings, passing them through the Introducer without problems, logging them correctly

Let's collect pointers to other tickets (especially the twisted ones) and progress reports here.