Add cli flag to start worker processes with daemon=False

[takhs91]

This allows the use of multiprocessing.Pool inside an actor as discussed in https://www.reddit.com/r/dramatiq/comments/fdus4w/support_for_cpu_intensive_workloads_and/

I initially ran the tests with daemon hardcoded to False and they passed but I made a cli option to reduce any chances altering the current functionality. I furthemore tried to analyze what changes it could create

Since you have already written sighandlers and logic about waiting the worker processes to exit using daemon=False should't leave any orphaned workers assuming cli executes normally and gets shutdowned by signals other than sigkill or sigquit(those also leaves the worker processes orphaned even with daemon=True it could probably only be solved from their side).

If an exception happens in cli before or inside the current process waiting code, it seem to be causing a deadlock in exiting in either way the daemon flag is set.

To conclude if you want to enable actors using multiprocessing I believe that this will cause no harm to existing functionality with either option set. Solving any of the afformentioned issues should probably be tackled in different issues. I would like to hear your thoughts!

[Bogdanp]

Thanks! Let's go the other route: might as well not add a new flag unless we need to. I'll go through some of my own test cases once you make the change.

[Bogdanp]

Since I closed the other PR, please add yourself to the contributors file here. Thanks again!