Foreign key constraints missing: holdings_record(instanceId) and item(holdingsRecordId)

holdings_record should have a foreign key defined on instanceId
Item should have a foreign key defined on holdingsRecordId

This is a bug fix and thus not intended to change the API - in a breaking or non-breaking way - but since applying those foreign keys would place conditions on the order of inserts into instance, holdings_record and item, it will probably require a major version upgrade anyway.