GeoTicketPlugin should cache unlocatable results

[Jeff Hammel]

locate_ticket is currently quite slow on the /query screen if there are tickets with unlocatable locations. This is because these locations are not cached:

    def locate_ticket(self, ticket):
        if ticket.id:
            results = get_all_dict(self.env, "select latitude, longitude from ticket_location where ticket='%s'" % ticket.id)
            if results:
                return ticket['location'], (results[0]['latitude'], results[0]['longitude'])

        if ticket['location'] is None or not ticket['location'].strip():
            raise GeolocationException

        # XXX blindly assume UTF-8
        try:
            location = ticket['location'].encode('utf-8')
        except UnicodeEncodeError:
            raise

        location, (lat, lon) = self.geolocate(location)
        if ticket.id:
            self.set_location(ticket.id, lat, lon)
        return location, (lat, lon)

A set of unlocatable locations should be hashed on the instance so that additional requests need not be made for bad locations (or more precisely, they need not be made more than once per instance).