This PR avoids a potential deadlock on shutdown of a Lean program when
the number of pooled threads has temporarily been pushed above the
limit.
There's a potential race between the finalizer "waking up everyone"
after setting `m_shutting_down = true` and a worker that is about to be
throttled because of concurrency limits.
- `m_max_std_workers = 1`, `m_std_workers.size() = 2`, and the queue
still has tasks.
- Finalizer sets `m_shutting_down = true` and calls `notify_all()` while
a worker is running a task (outside of the mutex).
- Worker finishes a task, re-enters the loop, sees work, and "should
wait" because `active >= max`.
- Worker then calls `wait()` after the notify and never wakes, so
`join()` in the finalizer hangs.
This PR avoids the worker being blocked by not `wait()`ing if we are
already shutting down. The code is restructured a bit for readability,
where the first section is "there's no work in the queue" and the next
section is "there is some work in the queue"
5d41b3bdce