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.
Since this is writing to the same table as all of App Fabric APIs,  do we need a separate store class? I had the same for Namespaces, but it adds a lot of code (the transactional objects in the constructor plus the ``NotificationFeedMds`` class) for just 4 methods. Can we move them to ``Store``, ``DefaultStore``? If we were to change anything in the Transaction system in future, we would have to change it in this class as well.