Notifications System core

Following https://issues.cask.co/browse/CDAP-849 jira and design doc.
Build is running at https://builds.cask.co/browse/CDAP-DUT320-3.
True, that class is very similar. I could even reuse the ``ServiceWorkerDatasetContext`` entirely in my code. I'll create another quick PR to just move it in a common space, and will merge to this PR once it's in develop.