[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


What we did for Spark Streaming is that each of the external connectors has a module called XUtils with static utility functions for creating things:

```
object KafkaUtils {
  def createKafkaStream(streamingContext: StreamingContext)
}
```

For this reason it might be good to call this `JDBCUtils` in similar fashion.