Jeff Parker once turned down a job from Joni Mitchell . It was early 1998, and the luck of the guitarist, newly 30, suddenly seemed to be shifting. He had been in Chicago for just less than a decade, cobbling together every gig he could find—weddings, funk bands and Motown acts, free improvisation with men hunched behind computers—to pay rent. But he had just joined the Association for the Advancement of Creative Musicians, a clearinghouse of adventurous jazz players and advocates for radical