Warn if a connected cluster is garbage collected, and disconnect it.

Suggested release note:

If a `Cluster` becomes eligible for garbage collection, and you haven't called `cluster.disconnect()` yet, the SDK now logs a warning and disconnects the cluster for you. This prevent the abandoned cluster's network connections from living forever. Please do not rely on this "auto-disconnect" feature. It's always better to call `cluster.disconnect()` as soon as you're done talking to the cluster, so the network connections and other resources get released right away.

If a user neglects to disconnect a Cluster prior to it being garbage collected, we'd like to log a warning and automatically disconnect it for them. Otherwise, the zombie core continues to consume resources.