TT#158252 GET /api/subscriberpreferences performance

* prepare_resource() use prefetch 'attribute' to address
  scenarios with high amount of preferences per subscriber

Change-Id: I7a32ff9953ccf9a3cc7e482639c6bd3cdb28eb0f

lib/NGCP/Panel/Utils/Preferences.pm

@@ -165,7 +165,7 @@ sub prepare_resource {
     }
     $prefs = $prefs->search({
     }, {
-        join => 'attribute',
+        prefetch => 'attribute',
         order_by => { '-asc' => 'id' },
     });