Signal handlers: Only cleanup current chirp instance
Signal handlers: Only cleanup current chirp instance

When we get a SIGINT or SIGTERM, we now only cleanup the
current chirp instance instead of them all. This simplifies
the code, as libuv will call the signal handler on all registered
chirp instances, thus freeing us from iterating with all it's hassles.

ganwell/chirp,ganwell/chirp,concretecloud/chirp,concretecloud/chirp,concretecloud/chirp,ganwell/chirp