Add visit properties to origin→snapshot edges

We should add visit metadata to origin→snapshot edges as labels, so that they can be used when manipulating the compressed graph only.

The following properties should be considered (from the schema of origin_visit_status):

• date: must have, as it allows to sort snapshots (possibly from the origin_visit table, see discussion)
• visit: optional, to be considered, as any easier to use alternative (it's just a small integer) than date (it's going away, see discussion)
• type: would allow to only care about some types of visit (e.g., git), but maybe it's too niche a use
• status: optional, to be considered, would allow to avoid following partial visits