rwsplit user executing large package sql should not be limited by the maxPacketSize set by Dble

[wenyh1]

1.读写分离用户，dble的maxPacketSize使用默认值4M，后端mysql设置max_allowed_packet为8M：

mysql> select @@max_allowed_packet;
+----------------------+
| @@max_allowed_packet |
+----------------------+
|              8388608 |
+----------------------+
1 row in set (0.00 sec)

2、jdbc读写分离用户执行6M的大包语句，返回报错

预期：执行成功
(association inner-2048)