LUSAKA—Zambian President Edgar Lungu conceded defeat on Monday, after a landslide election win by opposition leader Hakainde Hichilema, who will face the challenge of reviving an economy in turmoil. With all but one constituency counted, Hichilema had won 2,810,777 votes against Lungu’s 1,814,201, and the electoral commission declared him president. It marked the country’s third…