Brian Lamb, who created the revolutionary nonprofit cable television network C-Span in the late 1970s and has been its public face ever since, is handing it over to two lieutenants, Rob Kennedy and Susan Swain. Effective April 1, they will become the co-chief executives of C-Span and Mr. Lamb will become the executive chairman, formalizing a management change that has been years in the making. Mr. Lamb will continue to host “Q&A,” his Sunday night interview program, and will pursue other interests, like teaching