Optimization of DISTINCT SQL.

[tristaZero]

Optimization of DISTINCT SQL.

• Fix bug: can not executeselect distinct t_order.order_id from t_order;
• Support to use select distinct t_order.*, t_order_item.order_id from t_order, t_order_item where t_order.order_id = t_order_item.order_id

when there is * in sql, we need to get exact columns from shardingMetadata.

• Add integrated test cases of distinct SQL to cover SQLS mentioned above.
• Instead of columnLabelAndIndexMap, use QueryResultMetaData for DistinctQueryResult
• Instead of TreeMap, we should consider to use HashMultimap for QueryResultMetaData