1) Add a 1st product to compare from the ecommerce /shop page.
2) Open the compare list pop up at the bottom.
3) Add a 2nd product to compare from the /shop page.
4) Remove the 1st product from the compare list before the rpc of the
2nd product is done.
5) Javascript error `self.product_data[res] is undefined`

The function `add_new_products` and `rm_from_comparelist` are
concurrent, they both modify the internal `comparelist_product_ids`
product list. The first one perform an RPC and delay the update once the
request is done, the second does the update immediately.

The solution has been to use a mutex to exclude the two functions.

opw-1974108

closes odoo/odoo#33302

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>