Gets dict type value of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL

def db_get_map(self, table, record, column):
        """
        Gets dict type value of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL
        """
        val = self.db_get_val(table, record, column)
        assert isinstance(val, dict)
        return val