Immad Akhund, CEO and co-founder of the startup Mercury.Courtesy: Mercury TechnologiesMercury, a fintech firm that provides banking services to startups, has raised $200 million in funding at a $5.2 billion valuation, CNBC has learned exclusively.That valuation is 49% higher than the San Francisco-based company's previous funding round just 14 months ago, bucking the downturn facing much of the fintech sector.The Series D round was led by venture firm TCV — backer of other well-known fintech fir...