HDFS-4650. Fix a bug in FSDirectory and add more unit tests for rename with existence of snapshottable directories and snapshots.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1476012 13f79535-47bb-0310-9956-ffa450edef68