Add io_concurrency_limit support to prevent resource exhaustion

[iamthebot]

Closes #1475.

Problem: due to #1488 I'm having trouble actually testing this on linux... may need some help.

[iamthebot]

@wolfv I've tested this on OSX and it solves the problem. However, I'm not sure why CI for OSX is failing with the following error:

error: toolchain 'stable-aarch64-apple-darwin' may not be able to run on this system
note: to build software for that platform, try `rustup target add aarch64-apple-darwin` instead
note: add the `--force-non-host` flag to install the toolchain anyway

[wolfv]

I think the CI failure is solved in main (just remove this section).

I'll check it out tomorrow and merge :)