Simplified, improved RBAC storage backend.

Replaced rbac storage API with a simpler interface that will be easier
to implement over more backends (RDBMS, kv-stores, flat files, etc.).
Low-level storage is a simple knowledge base inspired by triplestores.

Groups as well as role grants can all be expressed in terms of this
simple storage API.

Updated the in-memory and MongoDB backends to implement the new storage
interface.

Relocate the storage implementations under rbac/storage. They're not
storage for anything else.