Auction.on_enter is not idempotent

https://gitlab.raccoongang.com/aliens/prozorro.sale/procedure/blob/master/src/prozorro_sale/procedure/state_machine.py#L122

This method is not idempotent and it may cause undefined behaviour in case procedure save will raise an error. We need to change it (and auction.databridge) in such way that only 1 auction could be created for a given procedure.