code: func (b *Backend) SetStatePending(signature *tasks.Signature) error {
  	update := bson.M{
  		"state":      tasks.StatePending,
  		"task_name":  signature.Name,
  		"created_at": time.Now().UTC(),
  	}
  	return b.updateState(signature, update)
  }
docs: // SetStatePending updates task state to PENDING