Blocker Description
KafkaController calls updateLeaderAndIsrCache() in onBrokerFailure(). This is unnecessary since in onBrokerFailure(), we will make leader and isr change anyway so there is no need to first read that information from ZK. Latency is critical in onBrokerFailure() since it determines how quickly a leader can be made online.
Similarly, updateLeaderAndIsrCache() is called in onBrokerStartup() unnecessarily. In this case, the controller does not change the leader or the isr. It just needs to send the current leader and the isr info to the newly started broker. We already cache leader in the controller. Isr in theory could change any time by the leader. So, reading from ZK doesn't guarantee that we can get the latest isr anyway. Instead, we just need to get the isr last selected by the controller (which can be cached together with the leader in the controller). If the leader epoc in a broker is at or larger than the epoc in the leaderAndIsr request, the broker can just ignore it. Otherwise, the leader and the isr selected by the controller should be used.
