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.
libtorrent < 2.0.7 has a bug that add_torrent_alert is posted too early
(before torrent is fully initialized and torrent extensions are created)
so we have to fill initial torrent data in add_torrent_alert handler.
PR #17491.
It did not work correctly, since it assumed that 'lt::torrent_plugin' is created at an earlier stage and is able to track all changes in the torrent state, but in reality it turned out that it was created after the torrent moved to the `downloading_metadata` state, so we had to additionally handle it in the constructor.
PR #16121.
"private" subfolders violate the structure of the project,
since the existence of all other subfolders is based on a
different principle.
In addition, there is no clear line between "private" and
"non private".
