Prevent infinite loop of OSM element fetching.

Keep a record of OSM elements fetched, to prevent dying upon
a circular dependency. This happens currently with e.g.
<http://www.openstreetmap.org/relation/3518103> and
<http://www.openstreetmap.org/relation/3490901>.
Don't cache when using local Overpass server.

This uses up a lot of disc space, plus the database is local anyway.