Remove duplicate subscribeForChanges in CallBackHandler

[xyuanlu]

Is your feature request related to a problem? Please describe.
During code inspection, We found there are multiple subscribeForChanges being called in CallBackHandler.handelChildChange. This leads to longer time spend when process callbacks in zkClient, witch eventually leads to increased PendingCallback queue size.

Describe the solution you'd like
Remove duplicate subscribeForChanges in CallBackHandler and refine the code structure to improve performance.