Fix task and model last_change handling

Improve db model index
Improve db model infrastructure

apiserver/schema/services/tasks.conf

@@ -501,6 +501,11 @@ _definitions {
                 type: string
                 format: "date-time"
             }
+            last_change {
+                description: "Last time any update was done to the task"
+                type: string
+                format: "date-time"
+            }
             last_iteration {
                 description: "Last iteration reported for this task"
                 type: integer