switch etag cache check to purely client-side

Instead of including the etag in the HTTP GET request and letting the server set the Response Code depending on whether the etag machines, this makes the client first issue a HEAD request, which is uses to get the etag and the file size. We need to do the HEAD beforehand anyway to get the file size for resumable downloads, and this approach prevents the server from using the etag as a form of tracking cookie: http://lucb1e.com/rp/cookielesscookies/

closes #562 (closed) !436 (closed)