HDFS-5535 subtasks:

    HDFS-5496. Make replication queue initialization asynchronous. (Vinay via
    jing9)

    HDFS-5645. Support upgrade marker in editlog streams. (szetszwo)

    HDFS-5752. Add a new DFSAdmin command to query, start and finalize rolling
    upgrade. (szetszwo)

    HDFS-5786. Support QUERY and FINALIZE actions of rolling upgrade. (szetszwo)

    HDFS-5753. Add new Namenode startup options for downgrade and rollback using
    upgrade marker. (szetszwo)

    HDFS-5835. Add a new option for starting Namenode when rolling upgrade is
    in progress. (szetszwo)

    HDFS-5754. Split LayoutVerion into NameNodeLayoutVersion and
    DataNodeLayoutVersion. (Brandon Li via szetszwo)

    HDFS-5848. Add rolling upgrade status to heartbeat response. (szetszwo)

    HDFS-5890. Avoid NPE in Datanode heartbeat. (Vinay via brandonli)

    HDFS-5869. When starting rolling upgrade or NN restarts, NN should create
    a checkpoint right before the upgrade marker.  (szetszwo)

    HDFS-5874. Should not compare DataNode current layout version with that of
    NameNode in DataStrorage. (brandonli)

    HDFS-5889. When starting rolling upgrade, create a fs image for rollback
    so that the standby namenode can create checkpoints during upgrade.
    (szetszwo & jing9)