Remove WorkerRun.worker

Closes #1371 (closed)

This causes changes on ListWorkerRuns, which is quite important to the frontend as it is used to list worker runs on processes. I optimized the queries so that I removed as many queries as this added. There are two extra queries only on CreateWorkerRun which could be optimized into other existing queries, but doing so requires too much work on the serializer so I didn't touch that.