A HEAD on a cache hit opened the stored object and closed it unread.
On S3 and Azure that is a GET whose body is thrown away, which can also
drop the pooled connection; on GCS it is a GET as well.
The server now marks HEAD requests in the request context, and readStored
answers a marked cache hit from the database record. Size, content type
and ETag already came from the record. Accept-Ranges depended on the
opened reader being seekable, so backends now say whether their readers
are (Blob does, GCS does not) and the HEAD response matches the GET.
A HEAD still counts as a cache hit, as before. DirectServe redirects are
unchanged since they are decided before the object would be opened.
