Return the last key/value pair from the given db.

def last(self, db=_DefaultDB):
        '''
        Return the last key/value pair from the given db.
        '''
        self._acqXactForReading()
        try:
            with self.xact.cursor(db=db.db) as curs:
                if not curs.last():
                    return None
                return curs.key(), curs.value()
        finally:
            self._relXactForReading()