Major Description
When submitting the AlterIsr request, we register a future listener to handle the response here. When receiving retriable error, we expected the AlterIsr request will get retried. And then, we'll re-submit the request again.
However, before the future listener got called, we didn't clear the `unsentIsrUpdates`, which causes we failed to "enqueue" the request because we thought there's an in-flight request. We use "try/finally" to make sure the unsentIsrUpdates got cleared (here), but it happened "after" we retry the request
Although the AlterIsr request will get sent next time when the follower sent next fetch request to the leader, we still need to fix this issue to make sure the AlterIsr request is sent successfully as we expected.