Hiring is surging in Silicon Valley, as nearly half of big U.S. tech companies - among them AAPL, GOOG and AMZN - increased employment by more than 50% in the last two years, and some small and mid-size businesses boosted payrolls nearly fivefold. But some tech veterans worry about Bubble 2.0, saying it’s starting to feel a little like the late 1990s tech bubble.