Test and document worker processes

[jmcarp]

The workers module doesn't have any unit or integration tests yet. We should write some, and continue to update tests when worker behavior changes. This might be a good task for the hackathon!

I'm also a little uncertain about exactly what the worker code does, and how it's organized. Better documentation might be helpful here, and might make it easier to write tests. Sequence diagrams could be especially useful--I like mermaid for this, but there are lots of great tools we can use to throw together a diagram in a few minutes.

[Frydafly]

Following up here since it's been some time! There's some description of the function of the workers module on here

@ibrand @jmcarp any ideas on how the existing documentation could be improved?