organize the task directory based on index

@xiang90 
I feel this will allow more uniform process of the tasks. what do you think?

Also there is any reason why
you want to arrange 
  /{app}/tasks/{taskID}/parentMeta
instead of
  /{app}/parentMeta/tasks/{taskID}
Can you reset the changes here?

I would like to discuss more on the framework change in another PR.