Quick fix for #405

[aclysma]

This is a quick fix for #405

• Use saturating_sub to avoid overflow in core assignment to task pools
  • This is likely a permanent fix
• Temporarily force 4 minimum threads to avoid examples stalling
  • It looks like thread pools are poorly behaved when they only contain a single thread. Forcing to 4 threads will get the examples running but likely the IO and async compute pools will not function properly. (Those pools are currently unused.) This is something to be addressed in a future change after having some more time to look into the root cause.

Tested on macOS by running locally:

• Naturally with many logical cores
• Forcing a max of 4 logical cores

[cart]

This is good to go, but lets add the // TODO like @memoryruins suggested