Etags/size in directory listing results don't match the files' MD5 when encryption is on

This is quite a tricky one... For HEAD requests we have access to the metadata of the file where we store the original MD5/size, but for directory listings it's not really possible. We could of course intercept it in flight and HEAD each file, but that'd quickly become very slow... Maintaining a local cache of the metadata would make things faster, but then we expose ourself to stale cache issues.
