[breaking] multiple refactorings in preparation for deferring torrent metadata resolution

[ikatson]

Full context for what problem is being solved is here: #287

Some torrents don't resolve metadata from magnet links instantly and might take a long time.

The problem with this is that (lib)rqbit's current architecture implies that the torrents persisted in session have metadata fully resolved and always available.

So today the session has no idea of torrents that are "being resolved". The use-case is - we want to be able to add a torrent to a session, and let it resolve the metadata in background.

There are 2 conceptual ways to address that:

1. Store "resolving" torrents completely separately from the the resolved ones. Then merge them together in UI, API, etc.
2. Update the current data model to support the new states and null-ness of metadata.

This PR prepares the code for the 2nd approach. Also some refactorings to increase maintainability of "add_torrent" and "start_torrent" code.