PR: https://github.com/truenas/middleware/pull/8170

This is to be merged for 22.MM, 22.02 performance fix will be proposed in a separate PR.

• By default `ThreadPoolExecutor._idle_semaphore` is set to 0, so `no_idle_threads` was
  always `True` and `IoThreadPoolExecutor` was never employed. Fix this in new `ThreadPool`

• Remove `ws_threadpool`, this is an artifact from the past. We already have a semaphore
  to prevent UI clients from calling too many methods simultaneously.

• Perform general cleanup.