We tend to lionize founders who start companies with the splashiest, most far-reaching consumer products. So Andrew Bialecki of $10 billion marketing tech startup Klaviyo—a name few people even know how to pronounce (it’s “clay-vee-oh”)—isn’t exactly a favorite to headline the next Code Conference.But Bialecki—a Boston-based, 37-year-old engineer who rarely gives interviews anyway—should be...