The way we query the DB in `submission_status` is prone to breakage

[SemMulder]

In common/submission.rs:submission_status we have the following comment:

// NOTE: The order is important here; a concurrent writer could move a submission
// from Paused to InProgress/Cancelled in-between the queries.
// from InProgress to Completed/Failed/Cancelled in-between the queries.

Instead of relying on our attention to detail. Let's rewrite the queries here to use a transaction and/or merge them into a single query using UNION ALL.

Found in #132 (comment).