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 NN startup options for downgrade and rollback using
    upgrade marker. (szetszwo)