Add multiple session detection, avoid multiple tabs

[qstokkink]

Fixes #8379

This PR:

• Adds an /info route to the EventsEndpoint.
• Fixes multiple tabs getting opened when double-clicking torrents, when an existing session is present. When not adding a torrent, a tab is still opened (otherwise, a remote session could lock you out from connecting to the core).
• Updates find_api_server to query the events endpoint and return basic session/events info.

[egbertbouman]

Couldn't you just make an (events) API request, after getting the url from server_url (like we do in start_download). Maybe this would have been less complicated.

[qstokkink]

@egbertbouman do you mean to /api/events? Assuming so, this was my initial approach as well, and the answer I found is: no, that is in fact more complicated.

All in all, the issue I found is that the events endpoint streams in a response. This is not the same as start_download. It means that we need to piece together the bytes response until we find the double-newline delimiter somewhere in the continuous stream and extract the first element, then close off the connection. It's possible, but requires active draining of a connection into some buffer. To avoid all that stream buffering and token parsing, I just added a new single-shot message, so we can use the new call just like start_download.

[egbertbouman]

Fair enough. I had a go anyway:

async with aiohttp.ClientSession() as session:
    response = session.get(server_url + "/api/events", headers={"X-Api-Key": config.get("api/key")})
    await response.content.readline()
    initial_message = (await response.content.readline()).decode()
    session_count = int(json.loads(initial_message[5:]).get("sessions", 0))

Of course this is without exception handling.

[qstokkink]

🤔 interesting code. I took some time to evaluate it, but ultimately I still think the implementation of this PR has a lower chance of scary things happening.