Critical Description
One symptom is that people connecting to Spark SQL through thrift server may see class not found exception when accessing their tables using custom serdes.
I feel https://github.com/apache/spark/blob/master/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/commands.scala#L103 is not enough. We probably should set the class loader explicitly to metadataHive like what we do at https://github.com/apache/spark/blob/master/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/commands.scala#L101.
