Only restock on_hand inventory units

Example: Spree restocks items once you cancel an order. However it
shouldn't add stock (restock) items from backordered inventory units.
Restock items but dont process backorders for canceled orders

Fixes possible issue where canceling an order with backordered inventory
units would make other orders inventory transition from backordered to
on_hand. e.g.

    - create two separate orders of the same item at the same stock location
    - both orders will have shipment state of backorder
    - cancel one of the orders
    - the second order inventory will transition from backorder to ready

Now the second order inventory remains backorder
Use flash.now for coupon errors

Fixes #4034