Multiprocessing integration

We do support `fork()` in general, but the multiprocessing module has some quirks.

* `atexit` does not get called, so events get lost
* Unhandled exceptions are not captured

An integration should fix both. We should also consider Billard, a fork of mulitprocessing here. If we manage to properly hook into both versions, we can catch some startup errors in Celery as well.

If you're interested in this, please give this post a 👍 so it's easier to track.