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_order.py
import logging
from datetime import timedelta
from functools import partial
import psycopg2
from odoo import api, fields, models, tools, _
from odoo.tools import float_is_zero
from odoo.exceptions import UserError
from odoo.http import request
import odoo.addons.decimal_precision as dp
_logger = logging.getLogger(__name__)
class PosOrder(models.Model):
    _name = "pos.order"
    _description = "Point of Sale"
    _order = "id desc"
    @api.model
    def _amount_line_tax(self, line, fiscal_position_id):
        taxes = line.tax_ids.filtered(lambda t: t.company_id.id == line.order_id.company_id.id)
        if fiscal_position_id:
            taxes = fiscal_position_id.map_tax(taxes, line.product_id, line.order_id.partner_id)
        price = line.price_unit * (1 - (line.discount or 0.0) / 100.0)
        taxes = taxes.compute_all(price, line.order_id.pricelist_id.currency_id, line.qty, product=line.product_id, partner=line.order_id.partner_id or False)['taxes']
        return sum(tax.get('amount', 0.0) for tax in taxes)
    @api.model
    def _order_fields(self, ui_order):