Reduce the total startup time of the application and maintain sufficient responsiveness of the UI during startup due to the following:
1. Load resume data from disk asynchronously in separate thread;
2. Split handling of loaded resume data in chunks;
3. Reduce the number of emitting signals.
PR #16840.
We can no longer save valid torrent files in the general case, because
for torrents of version 2, we need a full merkle tree to do it, but if
a torrent is added from magnet link, full merkle tree may not be available
even before the end of downloading all the data. Actually, we don't need
the full torrent file for the purposes of resuming the torrent, so we can
allow libtorrent to produce only a minimal part of the metadata as part
complete resume data, but we still want to store it in a separate file,
so we extract the resulting metadata from the complete resume data before
saving and merge it together before loading.
