BREAKDOWN OF HDFS-6581 SUBTASKS AND RELATED JIRAS

    HDFS-6921. Add LazyPersist flag to FileStatus. (Arpit Agarwal)

    HDFS-6924. Add new RAM_DISK storage type. (Arpit Agarwal)

    HDFS-6922. Add LazyPersist flag to INodeFile, save it in FsImage and
    edit logs. (Arpit Agarwal)
  
    HDFS-6923. Propagate LazyPersist flag to DNs via DataTransferProtocol.
    (Arpit Agarwal)

    HDFS-6925. DataNode should attempt to place replicas on transient storage
    first if lazyPersist flag is received. (Arpit Agarwal)

    HDFS-6926. DN support for saving replicas to persistent storage and
    evicting in-memory replicas. (Arpit Agarwal)