Provide Jython compatible ThreadPool implementation

[execveat]

Some Jython versions don't include multiprocessing.dummy module so the code at https://github.com/doyensec/GQLSpection/blob/dev/src/gqlspection/GQLSubQuery.py#L10 doesn't work in these environments.

We should provide a compatible implementation or find some other way to make concurrency work across Jython/Python2/Python3 environments.