Order/sequencing of holdings on an instance record. Re-order Holdings records

There are multiple potential designs for setting fixed ordering of holdings: Alternative 1: * Add ordering field to holdings record in back-end * When moving holdings to a new position, potentially all holdings in the list have to be updated - one by one - in the back-end with their new positions Variation: A new API is created that performs the multiple holdings updates behind the scenes in mod-inventory Alternative 2: * a new back-end structure is created that holds the ordering of holdings outside of the holdings themselves