kill subprocess_mixin children with parent

[wcpettus]

Any INT/TERM command to the top-level dragonfly process isn't propagated down to the children in subprocess_mixin. Because the default structure is a "control" subprocess and a "worker" subprocess that is actually handling the business, this results in two orphaned children that have to be cleaned up manually.

Our default process stop is a graceful INT, so on_channel_closed (https://github.com/project8/dripline-python/blob/master/dripline/core/service.py#L190) is called, which could attempt to cleanup any endpoints (if the subprocesses are added as endpoints)