parallelize tool may hang or crash

[gix]

Describe the bug
Adding batching of input files to the parallelize tool for my own needs (cuts the runtime for clang-format in half for me), I believe I encountered two issues:

1. Quite often it would crash upon returning from p.wait_all() since a kernel mode operation corrupted the stack. This goes away when using an explicit manual-reset event for OVERLAPPED in output_collecting_pipe.
2. It can hang on exit because the output_collecting_pipe destructor does not clear the running flag. The threadpool callback then starts another IO operation which never completes.

If useful, I can prepare a patch to fix these.

[StephanTLavavej]

A patch would be great - please submit a pull request when you're ready!

[StephanTLavavej]

I suspect this was the root cause of #257.

[BillyONeal]

I suspect this was the root cause of #257.

It might contribute but I seem to recall getting hangs even when we ran it serially?