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 it doesn't really matter, as per the design. But it's hard to unit test as it is now. As a workaround in the unit tests, I pushed one message on the feeds used in the tests, to ensure that we don't miss a notification.