Database Orders: Generate order numbers in the database

[duncanmcclean]

Right now, order numbers are generated by Simple Commerce before saving orders:

https://github.com/duncanmcclean/simple-commerce/blob/6.x/src/Orders/EloquentOrderRepository.php#L275-L288

However, if you have a busy site, you could potentially end up in a scenario where two users attempt to create an order at the exact same time and it'd error out due to duplicate order numbers.

For example...

1. Simple Commerce would give them both the same order number (the last order number + 1).
2. The first order would save in the database fine.
3. Any subsequent orders would error out when saving in the database due to duplicate entries for the same database column
   • Integrity constraint violation: 1062 Duplicate entry '1020' for key 'orders.orders_order_number_unique'

Ideally, for sites storing orders in the database, it would be great if we could push off the generation of the order numbers to the database so we don't need to deal with it in SC.

However, I need to do some further research into how we'd do that and if the same approach would work for all database engines, like SQLite.