only reverse propagate solo-by-others-downstream if connections on a Route change...

only reverse propagate solo-by-others-downstream if connections on a Route change, not just configuration changes.

This prevents a scenario where the route has had its solo control's state set from
XML, but then we configure the route's output, and invoke output_change_handler().
Without the new conditional, this would check which downstream routes are soloed,
find none, and then unset the soloed-by-downstream state of the solo control.