The “Booksmart” actor says his “cathartic” new role “felt like the right challenge at the right time.” After a six-year hiatus from the New York stage, Noah Galvin is delivering a dynamite performance in “ The Reservoir ,” an off-Broadway dramedy that explores alcoholism, aging and intergenerational relationships. Though the actor says the play “felt like the right challenge at the right time,” he had reservations about taking on a project in which several characters experience memory loss