HDFS-6346. Optimize OP_SET_XATTRS by persisting single Xattr entry per setXattr/removeXattr api call. Contributed by Yi Liu

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2006@1593581 13f79535-47bb-0310-9956-ffa450edef68