Change db_stress to work with format_version == 2

tools/db_stress.cc

@@ -1757,6 +1757,7 @@ class StressTest {
     block_based_options.block_cache = cache_;
     block_based_options.block_cache_compressed = compressed_cache_;
     block_based_options.block_size = FLAGS_block_size;
+    block_based_options.format_version = 2;
     block_based_options.filter_policy = filter_policy_;
     options_.table_factory.reset(
         NewBlockBasedTableFactory(block_based_options));