Add ETags for immutable directories

Add ETags for immutable directories

Like immutable files, the ETag is based on the storage index. However, since
a directory is a special interpretation of a file, it is distinguished from
the file by prepending "DIR:" onto the start of the ETag, and adding
-representation on the end (where -representation is the ?t= argument, json,
info, etc).

It also checks the return of setETag and avoids generating a representation
if the client already has it.