Fix concurrency issues with docdata calling status notifications.

- Both return URL and status-notification URL are called around the same time.
  However, in the return URL we would like to have the current situation,
  so we make a request to docdata and process it. Since Docdata may call
  the notification URL at the same time, this causes concurrency issues.
- Using `select_for_update()` now on the Order object.
- Using `transaction.atomic()` / `transaction.commit_on_success()` on
  the return views.
- Avoid sending signals when the status is already changed.
Use get_model() from django-oscar 0.7 when this is possible.
Truncate billingaddress.line1 to 32 chars in get_create_payment_args()