Properly handle ctrl-c termination of workers

[hellais]

Currently the workers for oonipipeline spawn a series of processes that then run async work inside of a set of worker threads.

For some reason getting them to nicely handle SIGINT is not easy. I spent some time debugging it, but still things were not working nicely.
It might be due to some bugs or inconsistencies in signal handling in python and async.

I am leaving here some of the links I read while debugging this:

• https://bugs.python.org/issue8200
• https://bugs.python.org/issue8296
• https://stackoverflow.com/questions/26413613/asyncio-is-it-possible-to-cancel-a-future-been-run-by-an-executor
• https://stackoverflow.com/questions/71132667/how-to-gracefully-end-asyncio-program-with-ctrl-c-when-using-loop-run-in-executo
• https://stackoverflow.com/questions/65832061/threadpoolexecutor-keyboardinterrupt
• https://stackoverflow.com/questions/76077605/handling-exceptions-and-ctrlc-in-python-multiprocessing-and-multithread
• https://stackoverflow.com/questions/11312525/catch-ctrlc-sigint-and-exit-multiprocesses-gracefully-in-python
• https://stackoverflow.com/questions/5045771/python-how-to-prevent-subprocesses-from-receiving-ctrl-c-control-c-sigint
• https://stackoverflow.com/questions/75680338/in-python-processpoolexecutor-do-you-need-call-shutdown-after-getting-a-brokenp

[hellais]

This is fixed in #81