[SPARK-2710] [SQL] Build SchemaRDD from a JdbcRDD with MetaData

SPARK-2710 Build SchemaRDD from a JdbcRDD with MetaData

and a small bug fix on JdbcRDD, line 109
it seems conn will never be closed


OK, I see, @yhuai actually added a TODO in https://github.com/apache/spark/pull/1346.
https://github.com/yhuai/spark/blob/dataTypeAndSchema/sql/core/src/main/scala/org/apache/spark/sql/json/JsonRDD.scala#L360