Revert "Revert "Eliminate stop_control atexit hook.  #91.""

This reverts commit fa11a2a2835322461809e2c0a969b24b9ba54c38.

The dual at_exit/on_cleanup mechanism tries to deregister twice on `touch
STOP`.  This could be fixed by adding e.g. a "stopped" flag to the
control object, but I think I'd rather fix this by just removing the
Pykka code, which has not ended up doing what I wanted it to do.

Conflicts:
	pipeline/pipeline.py