In many ways, Williamsburg’s newest venue couldn’t be more different from the (mostly) defunct DIY show spaces (bar/art-galleries and dingy old warehouses) that once lined the waterfront area. (Cameo, at least, is still here — for another month and a half, anyway). That’s because National Sawdust is a refined concert hall, a serious non-profit institution with powerful and moneyed supporters plus a leadership of established talent tapped directly from the music and art worlds.