fix: add _CloudPickleProcessPoolExecutor as default pool in FuturesExecutor

[lgray]

Fixes #1408

[nsmith-]

Can add Iason's test as well

[ikrommyd]

Thanks! I was not aware that you can subclass the process pool like that and define your pickler.

[lgray]

@nsmith- Yeah I'll add Iason's test once the baseline tests go. I probably typo'd something. Figured out a better way to set the pickler that makes fewer assumptions, added test.

[lgray]

Eh lots of stuff failing - I'll get it figured out.

[nsmith-]

If this doesn't pan out, another more brute-force option would be to create a payload wrapper that uses cloudpickle to serialize the actual work function and then give that to the process pool.