Optimize performance of ResultList

On at least one JDBC driver, returning a 10,000-row result set using ResultList typically takes 75-85s, whereas retrieving the same data using ResultSet takes only 10s. Optimize performance of ResultList so it is on par with direct ResultSet access.

Note that this will constitute an API change and slight loss of functionality (ResultList's iterator will only support forward-scrolling iteration, whereas previously it had supported bi-directional iteration). However, the performance benefits for the common case (forward-scrolling) should far outweigh the loss.