optimize Shipment#manifest
HACK: Fast insertion of inventory_units

This is a stopgap to allow us to process large orders in a reasonable
time. We have to replace this with something better (ie. models which
have a quantity associated)

For now this allows a checkout of 50_000 units in ~5 seconds. However,
the number of InventoryUnit records which are being created will quickly
get out of control.
fast Shipment#finalize!