[FIX] mrp: duplicate lot when auto-generate serial

Previous commit 1b840d06 tried to fix
the issue action_generate_serial may try to generate a existing lot.
But it made the lot number completely product specific. This change is
too big for stable, we revert the change in this commit.

Also to fix the issue, when action_generate_serial tries to generate a
lot, we always try the result from ir.sequence frist, only when the lot
already exists, we then try create one based on the product's lastest
lot number.

Task-3187003

closes odoo/odoo#120581

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>