- revise mapping of search terms to database types to match the
  GeoIP2 schemas.
- open GeoIP2 databases when starting up; close when shutting down.
- clarify the logged error message when an unknown database type
  is configured.
- add new geoip ACL subtypes to support searching for continent in
  country databases.
- map geoip ACL subtypes to specific MMDB database queries.
- perform MMDB lookups based on subtype, saving state between
  queries so repeated lookups for the same address aren't necessary.