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.

obsstore: add fields attribute to track each field in a marker We are going to increase the amount of data explicitly stored in obsolescence markers. This mean we are going to have a longer tuple and some values will be shuffled around. So we add a ``fields`` attribute to the obsstore class to keep track of what entry is what. This will be useful for extensions and for documentation purpose.

debugobsolete: explicitly display date in the output As the date is becoming a first-class citizen, we are displaying it in an explicit field. As a bonus it is now readable by humans.

obsolete: add a date argument to the `createmarkers` function The function is now just passing the value to create markers.

debugobsolete: use the new date argument on obsstore.create Now that we have this new argument, we can just use it.

obsstore: add an explicit `date` argument to obsstore.create The date will become an official field in the markers (and ultimately in the on-disk format). We start by making it an official argument for the function.