RBF: NamenodeHeartbeatService should update JMX report with configurable frequency

The NamenodeHeartbeatService currently calls each of the Namenode's JMX endpoint every time it wakes up (default value is every 5 seconds).

In a cluster with 40 routers, we have observed service degradation on some of the  Namenodes, since the JMX request obtains Datanode status and blocks other RPC requests. However, JMX report data doesn't seem to be used for critical paths on the routers.

We should configure the NamenodeHeartbeatService so it updates the JMX reports on a slower frequency than the Namenode states or to disable the reports completely.

The class calls out the JMX request being optional even though there is no implementation to turn it off:

// Read the stats from JMX (optional)
updateJMXParameters(webAddress, report);