WL#2232: Delay unpacking of rows, and use offsets array instead of individual byte[]s

The JDBC driver currently creates a new byte[] for each column value in a row.

For non-updatable result sets, it should be friendlier to the garbage collector
to wrap the Buffer that's read for each row with a Row class that stores the
_offsets_ to each column, and only materializes a byte[] when required
(getBytes() or getBlob()).