Feature: Fork kernel

[zmbc]

Building on work by @maartenbreddels and @edisongustavo in #410, this is my first pass at trying to add the feature I requested in #1236.

The tests pass 🎉 However, this involved a fair amount of trial and error and I still don't fully understand why it works the way it does -- see comments inline. I would really appreciate some ideas here.

Requires a corresponding change to jupyter_client: https://github.com/jupyter/jupyter_client/pull/1031.

[bluss]

fork() unfortunately interacts very badly with threading (OS threads). Is it possible that the user has kicked off some running threads?

See also discussion https://discuss.python.org/t/switching-default-multiprocessing-context-to-spawn-on-posix-as-well/21868