HDFS-6794: Merging r1615169 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1615175 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10900. CredentialShell args should use single-dash style. (wang)

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HDFS-6802. Some tests in TestDFSClientFailover are missing @Test annotation. Contributed by Akira Ajisaka.

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

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