Skip updating reference QA results if it was already done

If another workflow updates the same reference QA results between a workflow orchestrator running and its child work requests starting, then we can skip those work requests to save effort.

Fixes: #907 (closed)

As discussed on Matrix, I considered using a new work request status instead of a new work request result. However, that seemed to be a fair bit more involved because we'd no longer be able to take advantage of the existing logic in mark_completed most of which we still want to do (unblocking reverse dependencies, updating workflows), and the distinction between the two is otherwise quite marginal.