HDFS-4098. Add FileWithLink, INodeFileUnderConstructionWithLink and INodeFileUnderConstructionSnapshot in order to support append to snapshotted files.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1432788 13f79535-47bb-0310-9956-ffa450edef68