BOSTON — The Boston Symphony Orchestra is extending the contract of its music director, who last year became the orchestra's youngest conductor in a century.The orchestra announced Monday that 36-year-old Andris Nelsons' new eight-year contract will run through 2022. It replaces the original five-year deal that began in September.The new contract also includes an evergreen clause that reflects what the orchestra said was a "mutual desire" for a commitment beyond 2022.