Versions
--------
- 16.0+

Steps
-----
1. Have a cart ready to be paid;
2. open `/shop/payment` in two windows;
3. click "Pay now" in  one window;
4. click "Pay now" in the next window.

Issue
-----
Depending on version, installed modules & click speed, one of the following happens:
- Error: "The operation cannot be completed: Reference must be unique!"
- 500: Internal Server Error.
- Both payments get confirmed.

Cause
-----
There's no check on whether a payment is already being processed for a particular order.

This can happen in two manners:
1. The 1st one has finished, but the 2nd one is still allowed to start.
2. Clicking fast enough, both will try to transaction concurrently.

Solution
--------
1. If the `amount_total` is equal to `amount_paid`, raise an error.
2. Put a database-level lock on the Sale Order using `FOR NO KEY UPDATE` which will get released as soon as the payment transaction is committed to the database, or is rolled back due to failure.

opw-4555664

Forward-Port-Of: odoo/odoo#198873

### Steps to reproduce:
- Create a new invoice
- Attach a ZIP file then a PDF file
- The file viewer shows but tries to preview the ZIP file
- Happens with all non-viewable files

### Cause:
The ZIP file is put as main attachment and the method `register_as_main_attachment` only change the main attachment (with `force=False`) when there is no main attachment.

The behavior of the file viewer is problematic because we cannot switch between the attachments in the preview, so we cannot see the PDF. But if we add another PDF file (so one ZIP and 2 PDFs), we can switch but it will never show the ZIP again, only the two PDFs. This behavior is due to the [next/previous arrows](https://github.com/odoo/odoo/blob/e4da068d6c9c8885dd4663d50dee11c9ea1516a3/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.xml#L13) being displayed only if [`attachmentsInWebClientView`](https://github.com/odoo/odoo/blob/e4da068d6c9c8885dd4663d50dee11c9ea1516a3/addons/mail/static/src/models/attachment.js#L323-L328) contains more than one item. But this list only contains viewable attachments (PDF or Images), and the next/previous arrows only take attachments from this list. As the arrows are [changing the main attachment](https://github.com/odoo/odoo/blob/e4da068d6c9c8885dd4663d50dee11c9ea1516a3/addons/mail/static/src/models/web_client_view_attachment_view.js#L17) to change the displayed preview, it never comes back to the problematic ZIP file.

### Solution:
Also display the arrows if the main attachment is not viewable and there is more than one attachment.
This way the user can return in the list of viewable attachments.

This is not an optimal because Odoo will still try to display the ZIP file, but it is a simple fix that works. 
I tried restraining the main attachment field only to viewable files in this [PR](https://github.com/odoo/odoo/pull/196390), but it seems to break things specially with OCR extraction and apparently it is sometimes wanted to have an XML as main attachment.

opw-4486363

Forward-Port-Of: odoo/odoo#196446

In commit 2f62d5c0d78371be70586c79cb2b5931e733b042 the issue was fixed for some cases.

But the problem remains in some other cases.

In the original fix a special context value was added to the reconciliation of some lines. It prevents the creation of cash basis related moves for that reconciliation. This commit adds the same context to all the other reconciliations in the same function as the original reconciliation.

Reproduce on runbot for l10n_mx for bank payment method
1. Install l10n_mx
2. Set the 'IVA 16% VENTAS' tax as Customer Taxes on a product. Set the Sales Price to 100.
3. Create a payment method and journal for the PoS 
    * Payment method: Bank on journal BNK1
    * Journal: POS
5. Create a PoS using the payment method and journal from the previous step.
6. Start a PoS session
7. Sell the product from step 2 via the Bank payment method.
8. Close the Session
9. The following journal entries will be created: (All the tax lines use the same tax account; the "final" and not the caba transition account)
   ```
    * 1 entry in the POS journal (order)
       base:       | - 100.0 $
       tax:        | -  16.0 $
       receivable: | + 116.0 $
     * 1 entry in the BNK1 journal (bank / payment)
       bank:       | + 116.0 $
       receivable: | - 116.0 $
     * 1 entry in the CBMX journal (caba)
       base:       | - 100.0 $
       base:       | + 100.0 $
       tax:        | -  16.0 $
       tax:        | +  16.0 $
     * 1 entry in the EXCH journal (exchange difference)
       for the cash basis rounding difference
       tax:        | -  16.0 $
       tax:        | +  16.0 $
   ```
opw-4355124

Forward-Port-Of: odoo/odoo#199194