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. (szetszwo 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)

  HDFS-4097. Provide CLI support for createSnapshot. (Brandon Li via suresh)

  HDFS-4092. Update file deletion logic for snapshot so that the current inode
  is removed from the circular linked list; and if some blocks at the end of the
  block list no longer belong to any other inode, collect them and update the
  block list.  (szetszwo)

  HDFS-4111. Support snapshot of subtrees. (szetszwo via suresh)

  HDFS-4119. Complete the allowSnapshot code and add a test for it. (szetszwo)

  HDFS-4133. Add testcases for testing basic snapshot functionalities.
  (Jing Zhao via suresh)

  HDFS-4116. Add auditlog for some snapshot operations. (Jing Zhao via suresh)

  HDFS-4095. Add some snapshot related metrics. (Jing Zhao via suresh)

  HDFS-4141. Support directory diff - the difference between the current state
  and a previous snapshot of an INodeDirectory. (szetszwo)

  HDFS-4146. Use getter and setter in INodeFileWithLink to access blocks and
  initialize root directory as snapshottable. (szetszwo)