Provide rowsAs[T] rather than allowing each row to be converted

Currently have rows() which provides a Seq[QueryRow].  This requires a) converting the underlying QueryChunkRow into a QueryRow - GC pressure - and b) requires the app to handle errors converting each row, which is cumbersome.

I can't see a good use-case for wanting half the rows as one type and the other half as another.  So Scala will take a leaf from the Java client's book, and just provide rowsAs[T].