LONDON — As the rain at Wimbledon wreaks havoc with the schedule, and players start to complain, Andy Murray and Novak Djokovic have found little reason to worry about the weather. It helped that both got to play Monday on Centre Court, the only spot at the All England Club with a retractable roof. And with back-to-back, straight-set victories, they moved closer to a semifinal showdown that would be a rematch of the final last year, when Murray beat Djokovic to become the first British man since 1936 to win Wimbledon.