Store the MXF metadata in a hash table, keyed by the
instance UID. This simplifies resolval of the metadata
and makes looping over all metadata sets unnecessary
in most cases.
Additionally parse metadata always. If we already have
a metadata set with the same UID replace it only if
the new metadata set is from a later offset. This
fixes metadata parsing of files where following partitions
don't have a complete copy of the previous metadata.
