PR #17814.
Closes#17792.
Closes#929.
(Actually it should close all issues about lack of ability to stop torrent after metadata downloaded or after files are initially checked.)
Also makes explicit the temporary start of the torrent in the case when recheck of the stopped torrent is performed.
PR #17668.
This is a continuation of #17576.
Handles the case when you add hybrid torrent by v1-only magnet link and this torrent is already known by its full hybrid info hash (i.e. by both v1 and v2 info hashes).
