Provide ability for Python bindings to avoid `multiprocessing`

I am wanting to use the Python binding from a unikernel-type context -- the concept of processes doesn't apply, so the use of Python `multiprocessing` doesn't work here:
https://github.com/apple/foundationdb/blob/main/bindings/python/fdb/impl.py#L648

`threading` **is** available, so ideally that could be used instead? Perhaps selecting `multiprocessing` vs `threading` in a user-configurable way?