Rework new audiodb_track_id_datum_offset() and audiodb_insert_create_datum_offset() so that all offsets are in num_vectors. Internals of data storage are left the the methods to implement. If we are going to export these methods to the API, how do we expose the cache ? It needs to be passed back to the user and back in by the user on each call for caching to be re-entrant. No (intentional) functional changes this version.