fixes kubernetes-sigs#103

Creates a stop channel for the manager in New(), which will get passed to any
source.Channel instances that are added. When the manager's start method is
called and a new stop channel is passed in, that channel will be joined in a
goroutine with the manager's existing channel so that if the newer channel gets
closed, so will the manager's.

This cleans up, adds comments, and renames things in the stop channel
fix commits for clarity.

This will match the version currently used in Kubernetes.

Unparam is complaining of an usused param in cache.Reader in Travis (but
somehow only on Travis?).  This fixes that.