oak-core should check namespace registry consistency on startup

The persisted namespace registry contains maps from prefix to name and vice versa; plus all names and prefixes as multivalued string properties.

When rep:uris gets broken, ReadOnlyNamespaceRegistry may fail to check for duplicate namespace registrations .

We should check the consistency of the registry upon repo startup, and ERROR-level log issues for further analysis. Automatic repair might be non-trivial, though.

Seen in a real-world install: rep:prefixes containing 128 entries, but rep:uris a single one (with invalid value).