Add information about the configuration returned by the /admin/api/account/proxy_configs/ENVIRONMENT.json - either the last time it was changed, or some kind of hash that would identify the current state (e.g. the latest configuration versions of al services).

The purpose is to prevent APIcast from re-downloading and reapplying the configuration, if there were no changes since the last time the configuration was fetched - it might be an expensive operation, if many services are configured, and skipping this step would be useful.

We can consider returning a 304 status code (see https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/304) and use If-Modified-Since header, or adding this metadata in the response body or some other response header.

We need to make sure though that the information about the deleted service gets propagated successfully, i.e. a service is deleted between configuration fetches, APIcast needs to get the updated configuration and reconfigure the services.