Make sure that Tor doesn't build an IPv4 and an IPv6 connection to the same relay
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When we implement legacy/trac#17835 (moved), Tor will choose between IPv4 and IPv6 at random.
I'm pretty sure that we prefer existing connections to the same relay, even if the chosen address doesn't match.
But I need to read the client code to make sure this check is done on clients and relays. And I need to run the new code to make sure we're not doubling the number of connections that Tor clients make.