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.