std.Io.Threaded: when concurrent tasks caused pool to be oversize, reduce pool down to cpu count upon idling

[andrewrk]

Extracted from #25592.

Calling io.concurrent causes concurrent_count to increment, which can potentially make the thread pool larger than cpu_count. When a worker becomes idle, if it takes the concurrency count down enough, then the worker should exit so that the pool can downsize closer to the ideal setting.

            if (is_concurrent) {
                t.concurrent_count -= 1;
            }