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. 

addons/point_of_sale/models/pos_session.py
statements = []
        ABS = self.env['account.bank.statement']
        uid = SUPERUSER_ID if self.env.user.has_group('point_of_sale.group_pos_user') else self.env.user.id
        for journal in pos_config.journal_ids:
            
            
            
            ctx['journal_id'] = journal.id if pos_config.cash_control and journal.type == 'cash' else False
            st_values = {
                'journal_id': journal.id,
                'user_id': self.env.user.id,
                'name': pos_name
            }
            statements.append(ABS.with_context(ctx).sudo(uid).create(st_values).id)
        values.update({
            'name': pos_name,
            'statement_ids': [(6, 0, statements)],
            'config_id': config_id
        })
        res = super(PosSession, self.with_context(ctx).sudo(uid)).create(values)
        if not pos_config.cash_control:
            res.action_pos_session_open()
        return res
    @api.multi
    def unlink(self):
        for session in self.filtered(lambda s: s.statement_ids):
            session.statement_ids.unlink()
        return super(PosSession, self).unlink()
    @api.multi
    def login(self):