Implement namespace management

Our JCR implementation needs a namespace registry that stores the namespaces and their prefixes in such a way that they can be clustered. The best approach is probably to store the namespaces under "/jcr:system/" in a DNA-specific location (as the JCR spec does not specify that the namespaces are available as content. By doing this, the namespace registry would automatically work across a cluster (any clients that are using the same RepositorySource).