Fail fast(er) on JMX connection.

Improve behavior of concurrent `JMXNodeTool.instance()` calls when the JMX connection is unavailable.

This will prevent `connect()` from retrying forever, and also prevent repeated recreation of the connection without closing previous ones.

The vast majority of `JMXNodeTool.instance()` calls are as the result of an HTTP request, so retrying forever doesn't make sense there. The rest have their own retry loop (backups), time limit the connection anyways (checking if Cassandra is running), or run repeatedly anyways (ServiceRegistry management).