Enable clients to talk to public bridges via IPv6

We need to extend Tor for clients to talk to public bridges via IPv6. We may also need to: add an IPv6 GeoIP databases for bridge statistics, extend BridgeDB to give out IPv6 addresses of bridges, update metrics-db to sanitize bridge descriptors with IPv6 addresses. The exact steps will be described in the roadmap that comes out of legacy/trac#4556 (moved).

This ticket is a deliverable for June 30, 2012 for sponsor G.