Skaffold should provide a stopEvent

When Skaffold is started, it provides a start-event (though not programmatically identifiable):

```json
{
"result": {
"timestamp": "2020-01-30T16:56:47.256487Z",
"event": {
"metaEvent": {
"entry": "Starting Skaffold: &{Version: ConfigVersion:skaffold/v2alpha3 GitVersion: GitCommit: GitTreeState: BuildDate: GoVersion:go1.13.4 Compiler:gc Platform:darwin/amd64}"
}
}
}
}
```

But there's no explicit tear-down event. It would be useful for any clients to differentiate between the socket being dropped for some reason (unlikely as it is) vs Skaffold being stopped.