refactor the NewTreeTopology to use nTasks instead maxID
Merge pull request #74 from fengjingchao/master

Framework: clean up resources after task calls Exit()
Merge pull request #75 from xiaoyunwu/master

refactor the NewTreeTopology to use nTasks instead maxID
framework: naming, Exit -> AggregateJob
remove AbortTask
change tests to use AggregateJob instead of stop