Add country codes to relays inside a NetDir

• When the geoip feature flag of tor-netdir is enabled, perform GeoIP lookups for all relays added to the directory and add the resulting country code to the Relay struct.
  • The GeoIP database is provided in a new PartialNetDir::new_with_geoip constructor.
• A new trait was also added to tor-linkspec, HasCountryCode, to enable getting this data out from other crates.

Part of onionmasq#47 (closed).