`set_worker_thread_count` cannot be set to 1

What is the feature you'd like to have?
When we fixed #5170, we did so by preventing the worker thread count from being set to a value less than 2. We should revisit that, since my understanding is the change was made more for expediently resolving the situation than a fundamental technical limitation.

Is your feature request related to a problem?
A number of customers are trying to quickly process a large number of binaries in parallel and being able to confine each one to a single thread in headless is desirable for throughput. Additionally, being able to force things to be a little more deterministic might be helpful in certain situations.

Are any alternative solutions acceptable?
If I'm wrong and this is a more fundamental technical limitation, we should try and document why that is, prevent the setting from being set to 1 more obviously, and provide a warning if you try.

Start at the set_worker_thread_count entry point and review the change described in issue #5170. Determine whether a count of 1 can work for headless processing and, if so, define the expected behavior; otherwise document the technical limitation and ensure attempts to use 1 are clearly handled.