Consistently update `updated_at` for workflow status

[qianl15]

Currently, we update updated_at in insert_workflow_status and update_workflow_status, but we don't update this field in cancel_workflow or resume_workflow or other places.

We should consistently update updated_at across all operations.

Another issue is that if a workflow has completed but a later request invokes it again, the updated_at field will be updated even if the workflow doesn't run at all. We shouldn't update it if the workflow has completed.