Bitcoin topped $8,000 for the first time as investors set aside technology concerns that saw it tumble as much as 29 percent this month. The world’s largest cryptocurrency’s rise is becoming too big for many to ignore with three separate slumps of more than 25 percent all giving way to subsequent rallies, even as skeptics warn of a bubble waiting to pop. The digital currency is gaining the acceptance of professional investors, with CME Group Inc., the world’s biggest exchange, poised to start offering futures trading on bitcoin next month.