obsstore: drop 'date' from the metadata dictionary We are extracting the `date` information from the metadata at read time. However, we failed to remove it from the metadata returned in the markers. This is now fixed.

createmarkers: automatically record the parent of pruned changesets We need this information to build the set of relevant markers during exchanges. This can only be done at the `createmarkers` level since the `obsstore.create` function does not have a repo and therefore has no access to the parent information.

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".