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)

    HDFS-5535. Add BlockPoolSliceStorage 'trash' to handle block deletions
    during rolling upgrades. (Arpit Agarwal)

    HDFS-5494. Merge Protobuf-based-FSImage code from trunk - fix build
    break after the merge. (Jing Zhao via Arpit Agarwal)

    HDFS-5585. Provide admin commands for data node upgrade (kihwal)

    HDFS-5920. Support rollback of rolling upgrade in NameNode and JournalNodes.
    (jing9)

    HDFS-5945. Add rolling upgrade information to fsimage; and disallow upgrade
    and rolling upgrade to be started simultaneously.  (szetszwo & jing9)

    HDFS-5966. Fix rollback of rolling upgrade in NameNode HA setup.  (jing9
    via szetszwo)

    HDFS-5974. Fix compilation error, NameNodeLayoutVersion and
    DataNodeLayoutVersion after merge from trunk.  (szetszwo)

    HDFS-5963. TestRollingUpgrade#testSecondaryNameNode causes subsequent
    tests to fail. (szetszwo via Arpit Agarwal)

    HDFS-5976. Create unit tests for downgrade and finalize rolling upgrade.
    (Haohui Mai via Arpit Agarwal)

    HDFS-5980. Rollback does not need to load edits.  (jing9 via szetszwo)

    HDFS-5984. Fix TestEditLog and TestStandbyCheckpoints.  (jing9 via szetszwo)

    HDFS-5985. SimulatedFSDataset#disableAndPurgeTrashStorage should not throw
    UnsupportedOperationException. (jing9 via kihwal)

    HDFS-5987. Fix findbugs warnings in Rolling Upgrade branch. (seztszwo via
    Arpit Agarwal)