[PERF] mrp,stock,purchase: sql improvement

First correction: replace call to `read_group` to `_read_group` in order
to avoid useless joints on table product_template and ir_translation
when the grouping key is a Many2One.

Second correction: add an index on workorder's `state` field. The
get_conflicted_workorder_ids() method search for ready workorders that
are in conflict relate to their planning. The state value is searched to
many times to not index it

closes odoo/odoo#106536

Task: 2852423
X-original-commit: e94937b6
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>
Signed-off-by: William Henrotin (whe) <whe@odoo.com>