Add info about stopped/finished downloads in logs

[setaur]

Is your feature request related to a problem? Please describe.
I'm trying to build some kind of notification script (via telegram/email) for when a download is finished. If #2901 was finally introduced, that wouldn't be a problem, but without it I have to improvise something else.

Describe the solution you'd like
It would be very useful for me to have information about the finished download in the logs. When download is started there is a line in the logs, e.g.:

INFO:DownloadManager:Start handle. Download: Download <name: 'b'debian-11.2.0-amd64-DVD-1.iso'' hops: 0 checkpoint_disabled: 0>. Atp: {'save_path': '/home/bart/TriblerDownloads', 'storage_mode': libtorrent.storage_mode_t.storage_mode_sparse, 'flags': 208, 'ti': <libtorrent.torrent_info object at 0x7ffa5855c0f0>, 'resume_data': b'd11:file-format22:libtorrent resume file12:file-versioni1e9:info-hash20:\xc9\xa37V,\xb06\x0f\xd6\xf5\xab@\xfd+\x1b\x81\xd52]\xbde'}

But when download is finished, there is none indication about it in the logs. I don't think it would be a big burden to implement something like this? :)

[drew2a]

@setaur a PR with the requested feature will be merged soon.

Please take a look at the output:

INFO - Download(257) - On state changed alert: ubuntu-14.10-server-i386.iso: state changed to: finished
INFO - Download(408) - On torrent finished alert: ubuntu-14.10-server-i386.iso torrent finished downloading
INFO - Download(257) - On state changed alert: ubuntu-14.10-server-i386.iso: state changed to: seeding
INFO - Download(276) - On save resume data alert: ubuntu-14.10-server-i386.iso resume data generated.

[setaur]

Yeah, that would work great for me. Thank you!