[enhancement] Last announce time

[skobkin]

For now we have discovered_on timestamp.

But it would be good to have something like last_announced_on. It will help to decide which torrent to choose for more successful download as torrents which were announced a long time ago are more likely dead than newly reannounced.

It simply could be implemented by adding new method to the Database interface. Something like UpdateTorrentAnnounceTime() or something like that.