Background threads not being stopped during shutdown

It appears there are cases where calling `HostingEnvironment.InitiateShutdown()` will not stop all background threads. This means that if someone is in this situation and has exceeded the thread count threshold, they will infinitely repeat starting/stopping the host (due to the Health Monitor shutting it down). Only restarting the site manually will fix it.

Found this which has some examples: https://stackoverflow.com/questions/4432141/how-to-properly-unload-an-appdomain-using-c/5863164

I'm waiting on a concrete sample from this customer. When we have it, I'll do some more investigation into whether this is something we can help with.