obsolete: flip `obstore.successors` and `obsolete.precursors` People were confused by the fact `obstore.precursors` contained marker allowing to find "precursors" and vice-versa. This changeset changes their meaning to: - precursors[x] -> set(markers on precursors edges of x) - successors[x] -> set(markers on successors edges of x) Some documentation is added to clarify the situation.