obsstore: add a `parents` argument to obsstore.create We need a way to pass the information to the function. Some guru told me that what's arguments are made for.

obsstore: add a `parents` field This field is intended to store the parent of the precursor. This is useful to attach pruned changesets to a set of exchanged changesets. We currently just add the fields with a None value. None stands for "no data recorded".

obsstore: add some documentation about the marker fields As the number of fields grow, it makes sense to start documenting their contents.