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
for pos in self:
            pickings = pos.order_ids.mapped('picking_id').filtered(lambda x: x.state != 'done')
            pos.picking_count = len(pickings.ids)
    @api.multi
    def action_stock_picking(self):
        pickings = self.order_ids.mapped('picking_id').filtered(lambda x: x.state != 'done')
        action_picking = self.env.ref('stock.action_picking_tree_ready')
        action = action_picking.read()[0]
        action['context'] = {}
        action['domain'] = [('id', 'in', pickings.ids)]
        return action
    @api.depends('config_id.cash_control')
    def _compute_cash_all(self):
        for session in self:
            session.cash_journal_id = session.cash_register_id = session.cash_control = False
            if session.config_id.cash_control:
                for statement in session.statement_ids:
                    if statement.journal_id.type == 'cash':
                        session.cash_control = True
                        session.cash_journal_id = statement.journal_id.id
                        session.cash_register_id = statement.id
                if not session.cash_control:
                    raise UserError(_("Cash control can only be applied to cash journals."))
    @api.constrains('user_id', 'state')
    def _check_unicity(self):
        
        if self.search_count([('state', 'not in', ('closed', 'closing_control')), ('user_id', '=', self.user_id.id)]) > 1:
            raise ValidationError(_("You cannot create two active sessions with the same responsible!"))
    @api.constrains('config_id')
    def _check_pos_config(self):