Why there's not a classic thread pool

[tinylambda]

We have prefork, eventlet/gevent pools, and can run worker in solo mode.

it looks like that solo-mode worker is a single thread worker, but it's not really a pool. it does not support warm shutdown correctly(all tasks in solo-mode worker will shutdown immediately but not to wait them to finish when stopping the worker, event after setting CELERY_ACKS_LATE=True)

If we can have a classic thread pool, we can use a thread as main thread to control the pool, other threads will take the workloads.

[clokep]

I think there's a PR to add this, see #5099.

[tinylambda]

I think there's a PR to add this, see #5099.

Cool~