Thomas M. Menino, the longest-serving mayor in Boston history, died Thursday of cancer. He was 71.The affable Menino served as mayor for two decades, overseeing a vast redevelopment of underutilized corners of the city and ingratiating himself to residents with a perennially packed public schedule. (A Boston Globe poll once found that 57 percent of Bostonians claimed to have personally met him.)Menino left office earlier this year after declining to seek a sixth term due to ongoing health problems.