@Override
  public boolean isHardDown() {
    return hardDown.get();
  }
If the number of failures are above the threshold, the resource will be counted as down unless downUntil is in
the past.
Note how failureCount is not reset to 0 here. This is so that the node is marked as down if the first request after
marking a node back up, also times out. We only reset failureCount on an actual response, so down nodes get a
'chance' to prove they are back up every retryBackoffMs - they do not get 'fully up' status until they are actually
responsive.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/FixedBackoffResourceStatePolicy.java