[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


Move the schema stuff to `JdbcResultSetRDD`? We'd better keep the Spark core clean and same implementation pattern with the other Core RDDs.