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
- [x] Testing on a distributed cluster. 
So in case of a race condition, what would this do? Throw a TransactionFailureException? That is, to the HTTP client it will look like an internal error... I guess that should be handled with a retry, or with a returning a 409.