This commit concerns networks that use the --same-country option.
We recently discovered that the IP geolocation database contains
incorrect entries. To work around this, the protocol needs to be
changed by adding the country as 4th field in addresses (the first 3
are: ip, port, protocol) and the new --country option allows a node
to announce a country that differs from the one the GeoIP DB.

Thanks to the previous commits it's possible to implement backward
compatibility, by not sending the 4th field (country) to nodes that
can't parse it. Of course, these old nodes would continue to not
create appropriate tunnels and after a while, the administrator of
the network may decide to increase registry's --min-protocol (7).

In a network with only nodes that implement this last version of the
protocol, the nodes may only use the GeoIP DB to resolve their own IPs.

See merge request !27