For .net connector 6.8.8, can't connect to dble with the port 9066

[wjl1619]

• dble version：
  5.6.29-dble-9.9.9.9-5f335a2-20181127101819
• preconditions ：
  use .net connector 6.8.8 to connect dble with port 9066
• configs：

schema.xml

rule.xml

server.xml

• steps：
  step1.
• expect result：
  1.connect to dble successfully
• real result：
  1.2018-12-06 03:18:27.962 DEBUG [BusinessExecutor1] (com.actiontech.dble.net.FrontendConnection.query(FrontendConnection.java:303)) - [thread=BusinessExecu tor1,class=ManagerConnection,id=5,host=192.168.2.67,port=9066,schema=mytest] SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP())
• supplements：
  1.mysql> SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP());
  ERROR 1003 (HY000): Unsupported statement

/label ~BUG