plan: stop bundling geoip databases into the probe

[bassosimone]

This plan describes the required steps for stopping to bundle geoip databases into the probe. We previously discussed why this is problematic at #1954. A previous version of this plan can be found at #1954 (comment). This issue is an umbrella issue. The plan has been agreed upon with @hellais. Thanks to the users who prodded us to stop bundling geoip databases into the probe! <3

• Add support for returning probe_asn and probe_ip as part of check-in api#276 is merged
• when not using a proxy, the probe does not perform IP lookup and gets ASN and CC from the check-in API
• when using a proxy, the probe performs an IP lookup and passes X-Real-IP to the check-in API, which, in turn, causes the check-in API to use X-Real-IP rather than the user's IP for performing the geolocation
• we extend the web connectivity test helper to periodically fetch geoip databases
• we extend the web connectivity test helper to map all the resolved IPs (by the TH or by the probe) to ASNs
• we extend the websteps test helper to map all the resolved IPs (by the TH or by the probe) to ASNs
• we define a GeoIP databases update procedure for the check-in API
• we define a GeoIP databases update procedure for the test helper
• we drop support for performing GeoIP lookups from the probe

The above high-level plan needs to be converted into issues that are more fine grained.