geoip: add a geoip::data::lite class

Add a Lite data class that reuses a script from Debian to download the
GeoLite databases from MaxMind. This is useful for both databases that
have no paid-for equivalent (e.g. GeoIPv6, GeoLiteCityv6, etc.;
basically all but GeoIP.dat & GeoIPCity.dat) as well as for Labs, where
we can't distribute the proprietary databases.

Note that normally, the GeoIP.dat name is shared among both GeoIP
Country and GeoLite Country and hence would render this class in direct
conflict with the other data methods (while in practice, sometimes you
*need* to coinstall them). We diverge from what others do here, and
rename the Lite GeoIP.dat to GeoLite.dat. The naming conventions and
file paths under maxmind.com are really a mess anyway :)

Change-Id: I402d5dc56522f43c7f1c2c76fe04f98078b3f902