[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


user do not need to set parameters in SQL query if they only want to use 1 partition

such as i want to SELECT * FROM some_tbl

i do not need to write it like SELECT * FROM some_tbl WHERE 1=? and 2=? , then setLong(1, 1) and setLong(2, 2)