Close stale connections in standalone proxy

We've received several reports (#40211) of standalone proxies that have long-lived connections with clients but zero bytes transferred. The browser-based snowflake proxies (i.e., the web extension and badge) have a timeout in place to close stale connections after 30 seconds of inactivity. This aligns with a client-side timeout that closes stale connections to proxies after 20s of inactivity.

It's possible that the long-lived connections these standalone proxies are seeing are from clients not using our snowflake client code. Or that the client-side closures are not being received by the proxies. In any case, we should add an inactivity timeout to the standalone proxies to try and clean up these connections and free up resources in a similar way that the browser-based proxies do.