Inspired by #23373
The basic issue we ran into is that the loader is injecting globals directly into the global namespace. This means that these injected values are not thread or coroutine safe-- meaning we can never do more than one thing at a time. Instead of multiprocessing everything to death-- we can simply use a stack_context to nicely handle this down in the core. As far as the module authors/users are concerned nothing has changed-- but the storage behind the scenes is now per-JID. This same set of classes can easily be used to store additional data (next candidates are reactors, master MWorker tasks, etc.).
