Propose design for Dataset Handle (manager)

[ashao]

As decsribed in the design doc, the Dataset handle returned by the DataManager should be usable by the workflow itself to signal execution of tasks that depend on data availability. Open design questions are:

• How much of the state of a dataset should be held by this handle (update by callbacks) or rather computed based on the dictionary
• What is the minimal set of attributes and methods and what should be private and public/