Add workflow state timestamps to core table schema

In commerce_reports, we use the "created" timestamp on an order to produce sales reports. This is better than the other alternative, changed, which could be anything (moving from processing to completed for example). The problem with this is when someone creates an order 3 days ago and finally checks out, the sale goes into the report from 3 days ago. Ideally we'd store the checkout -> processing timestamp in the core schema ...