HIVE-6486 : Support secure Subject.doAs() in HiveServer2 JDBC client. (Shivaraju Gowda via Thejas Nair)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1576029 13f79535-47bb-0310-9956-ffa450edef68