CDAP-772 Feature/cdap 772 namespace service

Namespaces APIs to manage namespaces metadata.
Question: 
Should we create and use ``Id.Namespace`` instead of using namespace name as ``Key``?

- [x] Namespace service to create, list, delete namespaces
- [x] Unit testing, standalone testing
- [ ] Testing on a distributed cluster. 
Re-used an existing pattern for the name. Didn't expand the acronym. :)

I may have only partly addressed your previous comment. I have re-used the pattern in ``StreamMetaStore``. Like in ``StreamMetaStore``, I am re-using the table but not the ``Store``. Are you suggesting to add these methods to ``Store`` and ``DefaultStore`` instead?