[FIX] web: handle onchange on a on2many with an embedded one2many on multiple pages

Suppose there is a one2many field embedded in a one2many,
(say we are viewing record A with one2many field B itself with a one2many C).
Furthermore, there is a second page of the one2many field B.
If an onchange is applied on B, then the server replies with a 1 command for all
B records attached to the currently viewed object A.
This includes a list of commands [[5], [4, C_id]*] for each B record,
in particular B records that are on the second page.
Since they are on the second page, they might not have been fetched by the
frontend; in that case the corresponding C_ids are thus unknown.
As a consequence, on save the frontend sends a command 0 to create the records
for all unknown C_ids.
Since it doesn't have any value to give to the 0 command, the row creation
usually crash in the backend.

Another wrong behaviour is fixed at the same time: if the server sends an update
on the list of C ids with 4 commands.
In that case, if the record has not been prefetched, we have no way to know
if the list changed or not.

To solve this, when we receive the 4 commands by the server, we send back 4
commands.

coauthored by @aab-odoo

opw 1835936