Include subfile hashes with name lookup

[ewencp]

We currently get subfiles in headers, e.g.,

Subfile-Name: foo.jpg
Subfile-Path: /user/model/foo.jpg

Most of the time we're just going to download these anyway, so it would be nice to get all the hashes as well, e.g.,

Subfile-Hash: abcd6543

Especially when content is already cached, this could greatly speed up the client as it has to do many thousands of name lookups for dependent textures. Each of these is relatively quick, but we could entirely remove that overhead. It would probably be better for the CDN servers as well since they'll get far fewer http requests.