[FIX] point_of_sale: performance improvement to read one session

The fact the pos config name get was doing
`config.session_ids[0].state`
led to the prefetch of fields of all the session_ids of this config
on reading `config_id` for the session form

Therefore, if a pos config had 1000 sessions,
it was prefetching fields of 1000 sessions when you actually
wanted only one session to display in the form.
Among others, the `picking_count` field which is
not stored and quite heavy to compute

In term of time, it went from 1 minute to 1 second
to open the form view of one pos session.

addons/point_of_sale/models/pos_config.py

@@ -203,10 +203,11 @@ class PosConfig(models.Model):
     def name_get(self):
         result = []
         for config in self:
-            if (not config.session_ids) or (config.session_ids[0].state == 'closed'):
+            last_session = self.env['pos.session'].search([('config_id', '=', config.id)], limit=1)
+            if (not last_session) or (last_session.state == 'closed'):
                 result.append((config.id, config.name + ' (' + _('not used') + ')'))
                 continue
-            result.append((config.id, config.name + ' (' + config.session_ids[0].user_id.name + ')'))
+            result.append((config.id, config.name + ' (' + last_session.user_id.name + ')'))
         return result
 
     @api.model