Improve cpu_count by os.sched_getaffinity

### Problem Statement

I deployed self-hosted Sentry on a large-scale server. We split the server into many LXC containers. Then make docker nested in the LXC containers.

For example, I have a 112 cores physical server. Then I have a LXC container with 8 cores CPU limit in the server.
Then I deployed self-hosted Sentry in the LXC container.

The problem is that Sentry started so many workers that runs out so many CPU and memory.

I found that:

* Sentry thinks the CPU count is 112 rather than 8, so it started 112 worker processes.
* `docker exec sentry-worker-1 python3 -c "from multiprocessing import cpu_count; print(cpu_count())"` returns `112`.
* `docker exec sentry-worker-1 python3 -c "import os; print(os.cpu_count())"` returns `112`.
* `docker exec sentry-worker-1 python3 -c "import os; print(len(os.sched_getaffinity(0)))"` returns `8`.
* `docker exec sentry-worker-1 nproc` returns `8`.

References:
* https://bugs.python.org/issue36054

### Solution Brainstorm

I think we should replace all of `cpu_count()` with `len(os.sched_getaffinity(0))`.

### Product Area

Performance