[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


Not that there's anything wrong with backwards compatible fixes/enhancements, but a few things I noticed here:

1.  If it's a sufficiently small table that a user is only using 1 partition, why not encourage them to query it from the driver and broadcast it?

2.  As it stands, it looks like you allow 0, 1, 2, or more ? placeholders, but the doc comment change only describes the 0 or 2 case.