Provide "users-per-transport-per-country" statistics for obfsbridges

In our bridge-stats file we currently have bridge-ips which tracks the number of connections per-country, and bridge-ip-transports which counts the number of connections per-transport.

Still, these two data points don't allow you to infer the users-per-transport-per-country; which would give us useful information in case of blocked transports in specific jurisdictions, etc.

gamambel today suggested that we add such functionality, which seems like a marvelous idea.

wfn and grindhold seem interested in coding this. As discussed in IRC, interesting functions here are:geoip_get_transport_history() geoip_format_bridge_stats() validate_bridge_stats().

We should also define a nice format for this new line in bridge-stats. A not nice format is: bridge-ip-transports-per-country cn::obfs2:42,obfs3:46 ir::obfs2:10,obfs3:666 we might be able to find a better one (or even one that is used currently somewhere else in tor).