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.