DataNode启动优化改进：磁盘检测并行化
https://issues.apache.org/jira/browse/HDFS-8845 检查时不遍历所有子目录 https://issues.apache.org/jira/browse/HDFS-8850 VolumeScanner可能会抛nullpoint exception https://issues.apache.org/jira/browse/HDFS-7916 汇报坏块给standby namenode时一直出错
https://issues.apache.org/jira/browse/HDFS-8845 检查时不遍历所有子目录 https://issues.apache.org/jira/browse/HDFS-8850 VolumeScanner可能会抛nullpoint exception https://issues.apache.org/jira/browse/HDFS-7916 汇报坏块给standby namenode时一直出错