Violinist Jennifer Koh For the last subscription concerts of the season, the National Symphony Orchestra brought guest conductor Juraj Valčuha to the podium last night, in an alluring program of Haydn, Szymanowski, and Mahler. This concludes a two-year interim period before incoming music director Christoph Eschenbach takes the reins of the orchestra in the fall. The weekly game of “Who’s Conducting This Week?” has given the NSO faithful the chance to make the acquaintance…