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.
Unsure if this has to happen in this PR, since this PR already does a lot, but all non-CDAP-system entity APIs that have v3 APIs should start with have a prefix ``/v3/namespaces/{namespace-id}``.

If you can add an ``Id.Feed`` class and construct it in all the APIs below using ``{namespace-id}`` + a String ``feedId``, integration with namespaces will become much easier. You can look at ``Id.Program`` and model ``Id.Feed`` in a similar manner. Can be done in a separate PR though.