porch: additional concurrency tests

[johnbelamaric]

Describe your problem

In #3993 we added checks for resource version. This should help a lot, but it's probably not sufficient. For example, I suspect since we don't acquire a pacakgerevision mutex, you could still have a race condition where two simultaneous update calls came in and bypassed each others resource version checks. Similarly, I am not sure whether we have mutex on the package revision / package revision resources when we do our multi-commit saves (maybe we do have this?).

In any case, I suspect there is still a fair bit of work to do to make sure all concurrency issues are resolved, so I am raising this issue to track that work.

[johnbelamaric]

See also: #3993 (comment)