audio-sandbox: audio-graph-main updates.

- s/nodes/vertices/g, s/node/vertex/g
- Cleaned up the doc on dist_topo_sort_s slightly.
- The results from topo sort are now {dist, vertex} rather than {vertex, dist}.
audio-sandbox: dist_topo_sort - struct to class.
audio-sandbox updates.

- s/next_f/get_predecssors_f/g. This functor template parameter really expects
to get each vertex's predecessors, so the name was updated to reflect.
audio-sandbox: Doc update.
audio-sanbox: Changed an explicit weak_ptr::lock() to lock_ptr.