St. Petersburg. The very name brings to mind some of Russia’s greatest poets, writers and composers. Pushkin, Dostoevsky, Tchaikovsky. Others thought it was just too soon. Old, run-down Soviet Leningrad, they said, was not the St. Petersburg of 19th-century literature. What, then, is St. Petersburg In the confusing post-Soviet world, no one really knows. The quiet, if Soviet-style, dignity is gone. The revolutionary sayings are down, and gaudy advertising up. Candy bars and cigarettes are sold from boxy, tasteless kiosks. And clothing