Spend a few hours with Hollywood producer Jennifer Klein, and you might want to pop a Valium. Or slip her one. From the moment she rises at 7 a.m. in the Sunset Boulevard home she shares with her husband, she's a fidgety, demanding, chattering whirling dervish of a task juggler. Right now Klein, 41, whose credits include Pearl Harbor and Armageddon, has 15 film and TV projects in development--all