Expose request database weaknesses in unit tests

# Description

#493 highlights some weaknesses of the request queue logic. For example, stalled processes are never cleaned-up.

It would be useful to create a few unit tests that expose the problems discussed in #493 before implementing solutions.

For example, we could create a few dummy processes:
- completes after x second
- fails after x second
- hangs forever

Then launch these processes in different order and inspect the request database to see how it behaves.