[WIP] Stats

This adds a hard coded stats listener at `127.0.0.1:4000`.
The listener just accepts connections and sits there.
Chrome crashes the listener because we hit the assert.
There isn't a configuration syntax to make a listener, and I wanted feedback before I implemented it.

#### On the topic of configuration
With the existing configuration syntax, I think the easiest way to add stats would be:
```
listener 127.0.0.1:4000 {
    protocol stats
}
```

Stats isn't really a protocol per-se. I'm thinking it would be some sort of formatted text, maybe HTTP, JSON, who knows.

The alternative is to add a special case of listener like

```
stats 127.0.0.1:4000 {
  # optional body
}
```
I think the first option is easier in the short term.