The St. Louis Symphony Orchestra’s performance of Gustav Mahler’s Symphony No. 5 in C-sharp minor on Saturday, April 11 was one of those moments when it was good to be alive: it was good to be alive in St. Louis, it was good to be alive in St. Louis at this moment in the evolution of this orchestra, it was good to be alive in Powell Hall when the band was in town and Mahler’s 5th was on the conductor’s podium. Mahler’s 5th is a widely beloved pi…