[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


If you are are going to reuse the row object (which is a good idea), I'd use `mapPartitions` instead and create the object inside of the closure.