Make download service deal with HEAD efficiently

Currently a HEAD on a file download request is treated by the default servlet implementation, which consumes and throws out all the body just to compute a content-length (javax.servlet.http.HttpServlet.doHead and javax.servlet.http.NoBodyResponse.NoBodyResponse).

We can do better in the Nuxeo download service itself to avoid reading any bytes from the blob, which has a non-negligible cost especially if it's in S3 or similar.

Note that for CMIS this is already dealt with explicitly by the Chemistry framework, but this has to be refactored by NXP-27337.