[FIX] base: Open One2many with not yet updated values

When we have an onchange that modify items in One2many's lines (for
example the quantity of each line based on general quantity) and
we click on a record immediately after changing the field that
trigger the onchange, we have the the old value in the dialog box.
Indeed the dialog opens before the onchange has return the new
values, so the old one are used. This case appears only if the
One2many's line was never saved.

With this fix, we wait that the model is free of mutex, and so the
onchange is terminated before displaying it.

closes odoo/odoo#35306

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>