NamespaceRegistry should respond when stored namespaces are changed (in clustered environment)

At this time, the JcrNamespaceRegistry implementation does not change its cached set of registered namespaces when the content is changed. In a clustered environment, the namespace registry in one process will update the namespaces stored in the graph, but the namespace registries in the other processes within the cluster need to respond to those events and update their cached representations.

Note that, per Section 3.5.2 in the JCR 2.0 specification, the session-local mappings are initialized upon session initialization based upon the persistent (repository-scoped) namespaces. JcrNamespaceRegistry implements this behavior with two modes: one session-level behavior, and one repository-level (or workspace-level) mode. Really, only the repository/workspace mode needs to be changed (though I believe it essentially delegates to the namespace registry in the JcrRepository's ExecutionContext).