Revert HDFS-6788, bad merge.

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HDFS-6788. Improve synchronization in BPOfferService with read write lock. Contributed by Yongjun Zhang.

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HDFS-6793. Missing changes in HftpFileSystem when Reintroduce dfs.http.port / dfs.https.port in branch-2. Contributed by Juan Yu.

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Merge r1615270 from trunk. YARN-2343. Improve NMToken expire exception message. Contributed by Li Lu

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