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)

  HDFS-4077. Add support for Snapshottable Directory. (Nicholas via suresh)

  HDFS-4087. Protocol changes for listSnapshots functionality.
  (Brandon Li via suresh)

  HDFS-4079. Add SnapshotManager which maintains a list for all the
  snapshottable directories and supports snapshot methods such as setting a
  directory to snapshottable and creating a snapshot.  (szetszwo)

  HDFS-4078. Handle replication in snapshots.  (szetszwo)

  HDFS-4084. Provide CLI support to allow and disallow snapshot 
  on a directory. (Brondon Li via suresh)

  HDFS-4091. Add snapshot quota to limit the number of snapshots allowed.
  (szetszwo)