ctdldump: use the backend API instead of BDB calls

ctdldump: use the backend API instead of BDB calls

This makes ctdldump backend agnostic, or at least as backend
agnostic as citserver itself is at this point.  We will probably
add some calling syntax across the board when we add more
backends, but at least this utility doesn't have to be rewritten
now.  This makes me a happy ig.  ctdlload will be next.