Eight years ago, it was Melrose native Christopher Conway who needed help after a Christmas morning mishap caused his heart to stop beating. Only the fast action of family, friends, and paramedics saved his life. Now, his health restored, Conway has made it his life’s work to help others. A paid call firefighter in Center Harbor, N.H., the soft spoken 23-year-old on Friday took the national Emergency Medical Technician exam with the hope that certification as a basic EMT will give him a career boost. Conway wants to make the transition from call firefighter to full-time first responder.