With aggregate deal volume surpassing $3 trillion worldwide, 2015 has been another booming year for the global mergers and acquisitions market. But just how long will the party last?That’s the looming question in Dykema, Gossett PLLC’s 11th annual M&A Outlook Survey.This year’s survey respondents, a collection of top dealmakers, were less bullish, overall, when asked about the M&A market than they were in 2014, with just 37 percent expecting the market to strengthen in the next 12 months. However, …