Need a mechanism to detect and remove failed ZWave nodes

[mrstegeman]

The ZWave controller will eventually mark a node as failed if it stops responding after a while. We need a way to tag these nodes as failed, and a way to call the Manager::RemoveFailedNode C++ API.

I think that this can be reproduced by adding a node and then factory resetting it. You can then add the node a second time, and the first nodeId will stick around and should eventually get marked as failed.

See also: WebThingsIO/gateway#35