Keep history of mirror statuses

[stormi]

It would be nice to store a history related to the mirrors, to help detect mirrors that have frequent issues:

• up > down and down > up transitions
• if a trace file is used, how far behind they are at each synchronisation (if that's too much information, maybe just once every 3 hours)

[ott]

Would a log file suffice?

[stormi]

There already is a log, but it's just text so not that easy to use for this purpose.