Change migration to avoid tiggering callback and use private send:

* Skip :after callbacks on StockLocation and StockItem when creating
intitial instances. Regression due to 0ba60a08f934af9d6a7f17f6e9c90a1749547e9b and 630afbb7f2549556fb1bc914e16a2e3802cc8f15
* Use private send to set StockItem.count_on_hand
because :count_on_hand= is now private. Regression due to 9acf57f6baf55993ca0f4f032e9b7ad684712c9b

[Closes #2789]