Capital Group, one of the world’s oldest and most conservative investment firms, has become one of the biggest mainstream backers of bitcoin. What started as a $1 billion bet on companies holding bitcoin, has grown to over $6 billion, a major milestone in Wall Street’s relationship with the scarce digital asset. Mark Casey, an equity portfolio manager at Capital Group’s San Francisco office with more than two decades at the firm, has an interesting take on bitcoin. He’s a value investor in the Warren Buffett and Benjamin Graham mold, and has surprised many by being one of the most vocal …