Issue

For some reason, run_with_server.py barfs on macOS with Python 3.9 installed from Homebrew. If I run it, I get:

$ ./run_with_server.py --test-file r.txt  --concurrency 1
Placing 100 results in 20210415_224825_r
name 'session' is not defined
name 'session' is not defined
name 'session' is not defined
name 'session' is not defined
100%

and no saved results.

Looking at the code, it seems that on macOS, something is different about how global variables are inherited by the forked processes in the multuprocessing.Pool. On master, initialize(parsed_args) is only called once in the primary process, but it seems that all the workers fully inherit all the global variables. On macOS with Python 3.9, the global variables aren't present when the worker runs, and it barfs with the error printed above.

This PR makes a few slight modifications:

1. Switches to "proper" use of the initializer/initargs parameters to multiprocessing.Pool - each worker performs its own initialization call (on master, I think it's simply a mistake in invocation doing initializer=initialize(parsed_args) as initialize returns nothing).
2. Moves the multiprocessing.Value into the master process, and passes in copies to the workers.
3. Moves directory creation into the master process so it only needs to happen once.

With these changes, run_with_server.py works for me on macOS. Can someone on Linux please give this a shot and confirm it's still working there? Given the changes I had to make, I'm really not sure I understand how this was ever working on Linux - it seems to be some pretty fundamental difference in how multiprocessing.Pool is implemented.