[Improve] Connection pool refresh is supported

[wenyh1]

Add the following command to the administrative-side command to support connection pool flushing：

fresh conn [forced] where dbGroup ='dbGroupName' [ and dbInstance ='instanceName']
1、If dbInstance is not specified, the connection pool for all dbInstances under the default dbGroup is refreshed.
2、If the command contains forced, the connection in use should be discarded after it is close;
on the contrary, the connection being used and the corresponding front-end connection are all disconnect.