refine the task

Remove the Node related in task interface as they are framework implementation related. 
Remove BecomeMaster/BecomeSlave on the same ground.
Add getNode on framework so that task implementation can figure which node to talk to.

good question. I imagine for now just primary node. If we have hot standby, we should try to tee off traffic to hot standby automatically. But then again, we need to refine our thinking on that if we were to support that sometime.