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.

my thought is that `Node` is only used by the framework, so it is an internal stuff. A `Node` is a representation of physical computing unit. Probably we should make it private as `node`?