HIVE-6429 MapJoinKey has large memory overhead in typical cases (Sergey Shelukhin, reviewed by Gunther Hagleitner, Jitendra Nath Pandey)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1573404 13f79535-47bb-0310-9956-ffa450edef68