Make some json -> (text, xml) stuff in a common module, reference that in
account/container servers so we don't break existing clients (including
out-of-date proxies), but have the proxy controllers always force a json
listing.
This simplifies operations on listings (such as the ones already happening in
decrypter, or the ones planned for symlink and sharding) by only needing to
consider a single response type.
There is a downside of larger backend requests for text/plain listings, but
it seems like a net win?
Change-Id: Id3ce37aa0402e2d8dd5784ce329d7cb4fbaf700d
4806434cb0