Refactor stock related service objects to deal with inventory units.

Prior to this change, stock related service objects (coordinator, packer,
prioritizer, package, adjuster) were tightly coupled to orders and line
items, such that they would only create a set of shipments for the set
of line items for an order.

This change makes those objects work with inventory units, which enables
future enhancements that would create shipments for an order that are
not necessarily tied to the initial purchase (e.g. upcoming exchanges
work).
@huoxito you should be able to get the order from the contents no?  If it had to be added back for some reason it should probably just be an `options = {}` parameter so extensions can hook into overriding logic where necessary.