When subscription delivery is delayed (e.g., due to staggered
broadcast), the websocket connection could hit the idle timeout
before any data flows. This was because the data source
(including heartbeats) was not pushed downstream until the
client sent its first subscription message.

- Push the data source on the first downstream pull instead of
  waiting for a client message, so heartbeats flow as soon as
  the websocket connects.

- Emit a default heartbeat with a 5s step when no subscriptions
  are established yet. Once subscriptions arrive, the real
  step-based heartbeats take over on the next tick.

- Add debug logging for staggered broadcast delay.

Allow the set of cluster members to be updated even if the
downstream is still working based on the injected delays.