Ben Horowitz on the oldest law in startups, and why the industry no longer has to obey it: "The one thing we all knew in startup world is that if I have a two-year lead on you and you try to catch me by hiring 1,000 engineers, you're gonna wreck your company." "That never works. It's a Mythical Man-Month. Nine women can't have a baby in a month. That's it. That never works." "Okay, now that works. But it's not hiring 1,000 engineers. It's taking $3 billion and lighting up a magnificent…