hub: decorate Task.save with transaction.atomic

... to ensure that the task and its parent are updated atomically.

kobo/hub/models.py

@@ -605,6 +605,7 @@ def __str__(self):
             return u"#%s [method: %s, state: %s, worker: %s, parent: #%s]" % (self.id, self.method, self.get_state_display(), self.worker, self.parent.id)
         return u"#%s [method: %s, state: %s, worker: %s]" % (self.id, self.method, self.get_state_display(), self.worker)
 
+    @transaction.atomic
     def save(self, *args, **kwargs):
         if self.id is not None:
             self.subtask_count = self.subtasks().count()