Extend bridge statistics to learn about IPv4 vs. IPv6 usage

One of the main purposes for making bridges accept IPv6 connections was to circumvent some firewalls. But we don't have any statistics to see how well that works. IPv6 bridges always support IPv4, too, so we can't say from the current bridge statistics if connections came from one or the other address version.

In #5053 (moved) I briefly wondered if we should add a new country code "?6" to count unresolved IPv6 addresses. But that doesn't make sense, because as soon as we have an IPv6 GeoIP database, we wouldn't know if users coming from "de" actually connected to the bridge via IPv4 or IPv6.

How about we add a new line "bridge-ip-versions v4=NN,v6=NN" to count unique IP addresses for the two IP address versions?

In theory, we could add similar lines to the other statistics, but it mostly matters for bridge usage. I would want to do this for bridge statistics only, at least for the moment. And by "for the moment" I mean for 0.2.4.x.