svn merge -c 1501997 from branch-2.1-beta for HDFS-4908. Reduce snapshot inode memory usage.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1.0-beta@1502000 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1501993 from branch-2 for HDFS-4645. Move from randomly generated block ID to sequentially generated block ID.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1.0-beta@1502007 13f79535-47bb-0310-9956-ffa450edef68