Post-processing should be applied to generated holdings and item fields

The immediate requirement for this, like so much else, arises from the needs of the Able Bindery client. But it is fundamentally right that this should work. This was already discussion, and candidate solutions proposed, at https://github.com/folio-org/Net-Z3950-FOLIO/blob/master/doc/TODO.md#applying-post-processing-to-marc-records-with-holdings

See https://jira.indexdata.com/browse/DEVOPS-558