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 the state is still being held as a symbol, just cast to a strong for comparison purposes as an extra sanity check. moving the ContentItem into a more fully fleshed out class just allows the Package logic to be a bit simpler.