ETA calculation for seed_only torrents; ETA column sorting

Closes: #398

First commit makes it possible to calculate ETA for seeding torrents, based on current global (or torrent) MAX ratio.

Second commit fixes sorting by ETA column: sorting is done by internal qint64 value, not the display value. `∞` could correspond to multiple internal values: < 0 and >= MAX_ETA; we used -1 in `TorrentSpeedMonitor::getETA` and MAX_ETA in `TorrentModelItem::data`. This commit makes sure MAX_ETA is used in both cases.