reserve() {
        if (!this.config.reserved) {
            this.config.reserved = true;
        } else {
            logging.error(new errors.InternalServerError({
                message: 'Resource is already taken. This should not happen.',
                code: 'URLSERVICE_RESERVE_RESOURCE'
            }));
        }
    }
@description Reserve a resource.

This happens if a url generator's conditions matches a resource.
We have to reserve resources, because otherwise resources can appear in multiple url structures.