[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


i am afraid they do not think it is a problem, in the original comment of JdbcRDD:
```
 * @param sql the text of the query.
 *   The query must contain two ? placeholders for parameters used to partition the results.
 *   E.g. "select title, author from books where ? <= id and id <= ?"
```

but i believe many users just want to get the whole table out of RDBMS simply, and then do some calculation in Spark's magic world... how many partitions will be created is no matter, so in the normal use case, the tables stored in RDBMS are small, therefore these two ? placeholders for partitioning is not always necessary.