obsmarker: add `date` as an explicit field The markers are now 5-item tuples (concluded by the date). The obsstore.fields contents have been updated accordingly. There is no change to the on-disk format yet, so the date has to be extracted every time we read binary markers and re-injected each team we write them. This introduces a slowdown that will be solved when a new version of the format is added. Such a slowdown was already introduced by the evolve extension anyway.