Makes asynchronous logic to look more straightforward.
Allows caller to choose blocking or non-blocking way of obtaining asynchronous results via the same interface.
PR #22598.
Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).
Define "torrent ID" concept, which is either a SHA1 hash for torrents of version 1,
or a SHA256 hash (truncated to SHA1 hash length) for torrents of version 2.
Add support for native libtorrent2 info hashes.
