Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

PosSession = self.env['pos.session']
        closed_session = PosSession.browse(order['pos_session_id'])
        open_session = PosSession.search(
            [('state', '=', 'opened'),
             ('config_id', '=', closed_session.config_id.id),
             ('user_id', '=', closed_session.user_id.id)],
            limit=1, order="start_at DESC")
        _logger.warning('session %s (ID: %s) was closed but received order %s (total: %s) belonging to it',
                        closed_session.name,
                        closed_session.id,
                        order['name'],
                        order['amount_total'])
        if open_session:
            _logger.warning('using session %s (ID: %s) for order %s instead',
                            open_session.name,
                            open_session.id,
                            order['name'])
            return open_session
        else:
            _logger.warning('attempting to create new session for order %s', order['name'])
            new_session = PosSession.create({'config_id': closed_session.config_id.id})
            
            new_session.action_pos_session_open()
            return new_session
    def _match_payment_to_invoice(self, order):