feat: Add an option to configure subprocess pool size

[tonyo]

This can be useful when Symbolicator is not the only service running on the machine (e.g. in Kubernetes environment).

[jan-auer]

You will also need to update this here, which creates our standard thread pools:

symbolicator/src/utils/futures.rs

Line 53 in 2cad3b1

let runtime = tokio::runtime::Builder::new().build().unwrap();

[jan-auer]

Sorry, meant to request changes.

[tonyo]

@jan-auer you sure we want to have one setting for everything? Technically, there's also a worker pool in actix HttpServer where we can also adjust worker count
https://github.com/getsentry/symbolicator/blob/master/src/server.rs#L32

[jan-auer]

That's a good question. We may want to split this, indeed.

Some of these thread pools used to perform CPU-intensive work, but do no longer due to subprocessing. So in that case, we may consider to dial them down independently, so I think your patch is fine.

Should we maybe clarify in docs then that this is the number of sub processes?

[tonyo]

@jan-auer updated the docstring, but in the docs we already have this: