Avoid double notifying on node termination.

[kjnilsson]

When a node was terminated aten would sometimes send two notifications to registered listeners. One when it detected the node as not responding and one next interval when it detected it as gone.

This adds a check for gone case to see if the node has already been detected as down and only sends a down notification in this scenario if the prior state is not down.

Also return ignore if trying to monitor the current node.

[Zabrane]

When a node was terminated aten would sometimes send two notifications to registered listeners. One when it detected the node as not responding and one next interval when it detected it as gone.

This adds a check for gone case to see if the node has already been detected as down and only sends a down notification in this scenario if the prior state is not down.

Also return ignore if trying to monitor the current node.

@kjnilsson The fix works like a charm. I'm only getting 1 down message now and I can't register myself anymore.
Many thanks for your support.

[lukebakken]

@Zabrane thank you for testing this PR - it helps our team a lot.