JCR-Level 2: Support persistent namespace changes

JCR Level 2 requires support for persisting changes to namespaces in the repository. We can already support that with all of our connectors, since they're all able to store the namespace URIs for the content. In fact, none of them currently store or use the prefixes.

However, one thing DNA still doesn't have is the ability to manage the namespace prefixes in the connectors. This probably will entail having a NamespaceRegistry implementation that can use a connector to populate the namespace registry. This is a little tricky, tho, since the NamespaceRegistry is part of the ExecutionContext, which is passed to the connector's "execute(ExecutionContext,Request)" method. So, while it is ideal that these namespace management methods are just additional requests, the dependency graph may imply that the namespace management methods need to be added to the RepositoryConnector interface.