There was only one point when soul singer Rachelle Ferrell lost composure during last night’s gig at Yoshi’s Jazz Club in Oakland. It was towards the end of her set, and she was standing before a moderately-sized, but ecstatic and worshipful audience. Ferrell was about to sing the head of “Waiting,” a slow jam that usually begins with improvised melisma phrases over an extended vamp. “Oh shit,” murmured someone in the crowd, in response to the singer’s rich vocal adornments. “Shit,” he said again. A woman sitting two tables away overheard.