Clean GraphCDN response headers
Over time we cluttered responses from GraphCDN services with a bunch of unnecessary custom headers we use for debugging.
As of today, only the most important ones that are useful for debugging your caching behavior are sent back:
gcdn-cache: can either be HIT (was returned from cache), miss (should have been cached but wasn't), pass (should not be cached)
gcdn-scopes: the scopes that applied to the query
gcdn-scope-value-missing: whether at least one scope value required in the query was missing (i.e. whether this was treated as a "public" request)