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.
I originally had ``NotificationFeedStore`` implemented by App Fabric's ``Store``, but after discussing it with @chtyim, we decided to directly use the ``MetadataStoreDataset``, to remove dependency on app fabric in the notification module. It is true that it is a lot of code for only 4 methods. Not too sure what the best thing to do is.