if use uid IdGenerator, we may have incontinuous invoice number, but it seems it's very important to have continuous one.

the following scenario can reproduce error(in theory):

1. System call this script when user confirm invoice transaction, an invoice number is generated by portal_ids.generateNewId
2. If somehow this script ends with database conflit error
3. This script is called again, a new invoice number is geneated again

And uid IdGenerator use non_continuous_integer_increasing, there has no guarantee for continuous