SimpleSubscriberPlugin shutdown() implementation

[afrixs]

There is a TODO in SimpleSubscriberPlugin about waiting for ROS2 to support subscriber shutdown.

I believe ROS2 now supports subscriber shutdown - it's performed in subscriber's destructor.

I would propose to simply use impl_.reset(); like it's done e.g. in message_filters package. I've tested it and it works.

[Geoffrey Biggs]

Could you please submit a PR that includes a test? We can help you through the process if you're unsure how to do one.

[afrixs]

Reopening since calling publish on a disabled publisher caused process to crash (see #235)