[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


Would be good to print what the unsupported type is.  Also, try to wrap at the highest syntatic level, for example:

```scala
case unsupportedType =>
  sys.error(s"Unsupported jdbc datatype: $unsupportedType")
```

(Though actually in this case I think it'll all fit on one line).