Justin Timberlake, whenever he shows up to host SNL, can be counted on to deliver the goods, give 100 percent, and provide an installment not only worth watching, but worth recording and keeping. Someone else who deserves that sort of respect and anticipation, even if only after one turn as guest host, is Melissa McCarthy, who returns for her second stint tonight. Lorne Michaels would be smart to book her annually until she reaches that vaulted “Five-timers” club, and gets the same o