[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


This is not what your mean here. It is something like `SELECT * FROM tbl limit ?, ?`. But there is probably a bug in original implementation, as I remember the second parameter of `LIMIT` clause in MySQL is the `row_count`, not the `upper`. see: http://dev.mysql.com/doc/refman/5.0/en/select.html