pkg/watch: fix potential deadlock

I'm not sure it's actually a problem in the application, but I noticed a potential deadlock scenario with watch.Mux.  If a watcher stops reading from its ResultChan and calls Stop, the Mux could be locked in the distribute method attempting to send a new watch.Event to the watcher that's now no longer watching.
Oh, I see, if you do that then you have no way of knowing if it's even tried to distribute a message yet.