Map<Short, Map<Short, ContainerStorageStats>> aggregateHostAccountStorageStats(
      Map<Long, Map<Short, Map<Short, ContainerStorageStats>>> storageStatsMap) {
    Map<Short, Map<Short, ContainerStorageStats>> result = new HashMap<>();
    storageStatsMap.values().forEach(accountStats -> mergeAccountStorageStatsMap(result, accountStats));
    return result;
  }
Merge the given map from partition id to account id to container id to container storage stats, to a map from account
id to container id. If same account and container exists in different partitions, then just add storage stats up.
@param storageStatsMap The map of storage stats from partition id to account id to container id to storage stats.
@return The map from account id to container id to storage stats.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/MySqlClusterAggregator.java