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 don't think that it's true that in standalone we don't need to communicate through HTTP, because we need the feeds to be persisted, and we decided to persist then in app-fabric store. It's easier to go through the notification handler in app fabric, than to have a feed manager in standalone that gets injected all that's necessary to have access to ``Store``. Don't you agree?