Single request best effort diagram generation

Scenario:

Embedded PNG workflow diagram in a GitHub README file like:

https://view.commonwl.org/graph/png/github.com/common-workflow-language/workflows/blob/master/workflows/lobSTR/lobSTR-workflow.cwl

If the the workflow has never been seen before:
- Return image stating that the workflow is rendering
- In a separate thread, start workflow rendering and skip or defer the complete prospective RO processing that normally occurs.

If it has been seen before but the cached render is out of date
- Return most recent cached workflow image with an 'updating' disclaimer at the top
- In a separate thread, start the updated workflow rendering and skip or defer the complete prospective RO processing that normally occurs.

If it has been seen before and the cached render is up to date:
- Return the cached up to date rendered workflow diagram

Once this feature works there should be a widget to generate code for inclusion in multiple formats: plain HTML, MarkDown, ReStructuredText, etc..