Enable clients to talk to non-bridge relays via IPv6

We need to extend Tor for clients to talk to non-bridge relays via IPv6. This task depends on #4563 (moved). We may also need to update metrics tools like ExoneraTor to parse IPv6 in relay descriptors. The exact steps will be described in the roadmap that comes out of #4556 (moved).

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

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