[Improve]If there are too many datanodes(>700) for a few dozen datahost, start/reload will be slowly because of `testConn`

[yanhuqing666]

old way:
1.create conn for every data source for the test.
2.create conn for every data node of every data source for the test.
new way:
1.create conn for every data source for the test.
2.use show databases for every data source for the datanode test