add counter cache from variants to stock items

This adds a counter cache for stock items.

While running bullet and deleting some option types, it recommended me to setup a counter cache for stock items. I decided to do so. This was the result of that effort.
I think that would be a bad idea since postgres will still come back with a null value error. The concept is that you have to add the column, set the numbers for the rows, then set null to false. Reseting the counters will ensure all rows to to have an integer value.