Use country-specific speeds

Travel time estimation could be improved by taking into account local situation rather than global defaults.

Current solution of country-specific speed limits is based on the presence of the tag `zone:traffic` whose coverage is limited mostly to [Germany and Belgium](https://taginfo.openstreetmap.org/keys/zone%3Atraffic#map).

The information to which country a given way belongs to is already available in our external storage. A possible improvement could be based on the [OSM implicit maxspeed](https://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Maxspeed) in combination with [worldwide legal limits](https://en.wikipedia.org/wiki/Speed_limits_by_country). For a given highway type the most common value across countries could be used as the default and only outliers stored.

The actually tricky part is distinguishing whether a road is in an urban area. This cannot be inferred from `highway` type and there is no common global tagging for it.