Fixed concurrency problem in HiveHistory.java (contributed originally by @kayousterhout)
Fix bug of boolean equals() method, the reference comparison works incorrectly after deserialization.

Originally contributed by @chenghao-intel
Fix NullPointerException for Constant Null value

Originally contributed by @chenghao-intel
Reduce lock contention in ExprNodeGenericFuncEvaluator and ShimLoader.

Originally contributed by @chenghao-intel.
Merge pull request #46 from rxin/fixes

Apply some important fixes again for Hive 0.11