Return a 'retry-after' header when the request is rate limited.
See https://github.com/3scale/apicast/issues/335#issuecomment-427361128 for context.
Requires fix in apisionator, see https://github.com/3scale/apisonator/issues/54
This is proposed to be included as Technical Preview in 2.4. It will have to be included in the release notes. No visual element to flag as such. I suggest to create a KCS article for this.