varnish: switch from libGeoIP to libmaxminddb
This switches our Varnish from the old libgeoip to libmaxminddb,
which uses the new GeoIP2 databases and adds IPv6 support to the
GeoIP Set-Cookie and our geoiplookup JSON.
The region identifier is different than the old format, which may
cause some breakage; the old format was a numeric identifier in
the database, while the new format is a GeoIP ISO code.
Also, the final element of the cookie no longer differentiates
IPv4 vs IPv6 (which was primarily for related fallback), so the
final field is always 'v4' now.
One notable deficiency here is that updated MaxMind databases are
only reloaded when VCL is reloaded, but I think we can save that
problem for later varnish4/vmod work.