Boston hedge fund Highfields Capital Management is returning billions in client money and converting into a family office, founder Jonathon Jacobson told investors in a letter Wednesday. The decision to return money to investors marks one of the largest hedge fund closings in recent history. Mr. Jacobson started the $12.1 billion investment firm in 1998 after he left his post trading stocks for Harvard University’s endowment. Harvard was the first client at Highfields, which has about $9.5 billion in outside client money.