Use the new object request tracking mechanism for handling a
notify_ack request.

Move the callback function below the definition of this so we don't
have to do a pre-declaration.

This resolves:
    http://tracker.newdream.net/issues/3754Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>