Expose Backend container info on deleted containers.

Include basic container info in backend headers on 404 responses from the
container server.  Default empty values are used as placeholders if the
database does not exist.

Specifically the X-Backend-Status-Changed-At, X-Backend-DELETE-Timestamp and
the X-Backend-Storage-Policy-Index value will be needed by the reconciler to
deal with reconciling out of order object writes in the face of recently
deleted containers.

 * Add "status_changed_at" key to the response from ContainerBroker.get_info.
 * Add "Status Timestamp" field to swift.cli.info.print_db_info_metadata.
 * Add "status_changed_at" key to the response from AccountBroker.get_info.

Change-Id: Ie6d388f067f5b096b0f96faef151120ba23c8748
Merge "Expose Backend container info on deleted containers." into feature/ec