Updated CHANGES.txt to add HDFS-4033.


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HDFS-4034. Remove redundant null checks. Contributed by Eli Collins


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HDFS-4035. LightWeightGSet and LightWeightHashSet increment a volatile without synchronization. Contributed by Eli Collins


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svn merge -c -1430507 . for reverting HDFS-4353. Encapsulate connections to peers in Peer and PeerServer classes


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svn merge -c -1428729 . for reverting HDFS-4352. Encapsulate arguments to BlockReaderFactory in a class


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