Rename the 'ts' argument of 'origin_visit_add' to 'date'.

Summary:
To be consistent with the name used in the DB and the dict
key returned by 'origin_visit_get'.

Note that this makes the client incompatible with older
versions of the server.

Reviewers: Reviewers, olasd

Reviewed By: Reviewers, olasd

Subscribers: olasd, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D636