Prevent Invoice Numbering Clash Errors

Admins can choose the next invoice number or credit note number, but also then reset the next number.

This leads to the potential of there being a clashing invoice number, which would not be supported, so Upmind would throw an error.

We need to performantly check that there isn't a clash, and skip invoice numbers until there is no longer a clash. That results is invoices and credit notes being generated properly, without errors.