Regularly send scrape request to trackers

[chros73]

By default, rtorrent only sends scrape request (that can give back the numbers of peers, seeds, downloads for a specific torrent) to trackers when a torrent is added or the client was restarted. More info here: rakshasa/rtorrent#413
Probably this info only cosmetics but good to see these changing.
Use a custom field (tm_last_scrape) to store the last scrape time to be able to save into session (the inbuilt t.scrape_time_last property is lost upon reboot).
Let's send these requests regularly:

• try to balance the calls to not fire up them at the same time
• every 10 minutes for transferring torrents (uploading/downloading)
• every 12 hours for non-transferring torrents