Maintain daily country information for Onionperf guards

Currently guard country information is looked up in Onionoo at analysis time, as we have no historical IP geolocation database. This is fine when the measurements and the analysis are perfomed together, but re-analysing the data at a later date would result in the country information being very out of date.

A solution would be to produce another file at analysis time, containing the fingerprint to country mapping that can be re-used for future analyses.