Bug fix: Network Explorer: Add freegeoip API key and split out tasks for country distributions

[mmsinclair]

This PR fixes up a bug in the country distribution tasks and hopefully improves the code structure by splitting out the tasks more.

• freegeoip requires an API key for 15,000 free requests per hour (and the API hostname has changed)
• country distribution is in a separate task that runs every 15 mins
• mix node cache refresh occurs every 10 minutes
• geolocation runs as a permanent task that picks off a mix node that hasn't been located (or where the location cache time has expired - 1 day) and is rate limited to run every 50ms (a variation on the suggestion by @futurechimp )
• error reporting has been improved by adding more Error types to the location Result to log when the explorer-api is being rate limited by freegoip and also when a mix node's IP address cannot be located (there are a few that can't be found)

Things that could be improved / ideas / concerns:

• the task timers (e.g. mix nodes refreshed every 10 mins, distribution every 15 mins, location cache TTL of 1 day) could be tweaked / fiddled with because I just picked some numbers out of the air
• the code structure can be improved, apologies in advance for my bad rusting
• the geolocation job could create a queue of the mix nodes to locate and pop them off, instead of the for (...) { ... return; } pattern (a throw back to my C++ days)

I am much happier with the location task picking things off every 50ms because it works better with the naive way we spawn tokio tasks. The service will actually stop now when a halt signal is sent.