[FIX] core: optimize first snaphot in onchange

This patch speeds up the performance of method `onchange`.  Our use-case
is an invoice with 300 lines, where we modify the unit price or quantity
on several lines.  Each modification triggers a call to `onchange` on
the invoice itself.  The latter call went from 1.3 to 0.8 seconds, which
represents a speedup of 30% to 40%.

In the implementation of `onchange`, the first snapshot is preceded by a
"prefetching" phase, where the lines of x2many fields are read, so that
the fields of unmodified lines are in cache.  This is useful because the
fields of those lines are not sent by the client, which only sends ids.
This prefetching represents more than 40% of the duration of `onchange`,
in our use-case.

The prefetching is inefficient for several reasons.  First, it uses
`mapped`, which formats data that is actually never used.  Second, it
accesses new records that have the actual lines as origin.  And on those
records, computed stored fields are not taken from the origin record,
but are (uselessly) computed instead.

We have optimized the prefetching in the following way.  It now reads
stored fields on the actual lines (using `_read` to avoid formatting),
then copies the cache of those fields on the corresponding new lines (to
avoid useless computations).  This makes the prefetching about 10 times
faster!

closes odoo/odoo#59627

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>