HBASE-8662 [rest] support impersonation

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HBASE-8662 [rest] support impersonation - ADDENDUM

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HBASE-8662 [rest] support impersonation - ADDENDUM 2

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Fix mistake spotted by Benoit

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HBASE-8944. Document workaround for MiniDFSCluster exceptions when running tests

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HBASE-8953 TestFromClientSide.testClientPoolThreadLocal fails; DEBUG

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