[FIX] purchase_stock: _compute_on_time_rate optimizations

Turn the call to filtered after the search on purchase.order.line
into a call to _search. This allows to remove filtered by adding
an additional leaf in the search domain.

Add read calls to fetch fields from db and store them in cache
on recordset batches.

Example speedup: partner with 136555 POL 6.33s -> 3.42s

opw-3277299

closes odoo/odoo#120658

Signed-off-by: William Henrotin (whe) <whe@odoo.com>