ClearSnapshot should clean up watches

https://github.com/envoyproxy/go-control-plane/blob/52841810f634e67b22441ee50006655abadbc99c/pkg/cache/v3/simple.go#L264-L270

ClearSnapshot deletes the status for a node, but the states can have watches in it that will now never get triggered. Perhaps the right approach is to close the watch channels. However, looking at the SOTW server, closing any of the watch channels will make the stream handler error out.

So it's not obvious to me what the behavior of watches should be when a snapshot is cleared, but it seems something should happen since a subsequent SetSnapshot would not trigger any watches.