Branch-2802 Snapshot (Unreleased)

  HDFS-4076. Support snapshot of single files.  (szetszwo)

  HDFS-4082. Add editlog opcodes for snapshot create and delete operations.
  (suresh via szetszwo)

  HDFS-4086. Add editlog opcodes to allow and disallow snapshots on a 
  directory. (Brandon Li via suresh)

  HDFS-4083. Protocol changes for snapshots. (suresh)