Data store: Provide cursor-based data change API

Current DataChangeListeners incur heavy performance overhead, as each change is traversed and analyzed for change. As it turns out, applications usually need to extract only partial change information.

While BUG-2672 is the long-term solution, we need a stop-gap for Lithium, where the application will have control over how granular the analysis is. For this we need to provide a cursor-based API, on top of which the DataChangeListener API will be implemented.