Add a server parameter config option for number of workers/threads

The server that I'm running the experiment server from will kill my processes if they are using too much memory/CPU, which I find is the case if PsiTurk launches `2n-1` processes. This pull request adds a new server parameter, `num_workers`, which defaults to using `2n-1` processes but can be changed to specify exactly how many processes to use if you want.

I'm not sure if there was any other place that I should update to account for a new parameter -- let me know if I missed something, and I'll update the pull request.