Add an option for lazy subscription on scan topic

[bjsowa]

I use rplidar node with auto_standby option which stops the LIDAR motor when there is no subscription. Using it together with a laser filter forces the motor to constantly run as the filter chains constantly subscribes on the scan topic, even when there is no subscription on scan_filtered topic.

I think it would be beneficial to implement lazy subscription using rmw matched events, similar to how it's done in ros-perception/image_pipeline#815