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
from odoo import api, fields, models, SUPERUSER_ID, _
from odoo.exceptions import UserError, ValidationError
class PosSession(models.Model):
    _name = 'pos.session'
    _order = 'id desc'
    POS_SESSION_STATE = [
        ('opening_control', 'Opening Control'),  
        ('opened', 'In Progress'),               
        ('closing_control', 'Closing Control'),  
        ('closed', 'Closed & Posted'),
    ]
    def _confirm_orders(self):
        for session in self:
            company_id = session.config_id.journal_id.company_id.id
            orders = session.order_ids.filtered(lambda order: order.state == 'paid')
            move = self.env['pos.order'].with_context(force_company=company_id)._create_account_move(session.start_at, session.name, session.config_id.journal_id.id, company_id)
            orders.with_context(force_company=company_id)._create_account_move_line(session, move)
            for order in session.order_ids.filtered(lambda o: o.state != 'done'):
                if order.state not in ('paid', 'invoiced'):
                    raise UserError(_("You cannot confirm all orders of this session, because they have not the 'paid' status"))
                order.action_pos_order_done()
    config_id = fields.Many2one(
        'pos.config', string='Point of Sale',