The multiplayer server should not allow multiple connections from the same user on different devices

[LeNitrous]

Describe the bug: Having the same user connected on multiple devices will cause the multiplayer server to give the user an invalid state.

Reproduction Steps
Upon testing, device A was a PC while device B was an Android.

1. Create a multiplayer lobby on device A
2. Launch osu! on device B
3. Try updating settings (such as beatmap) on device A's lobby (an error should throw saying the user is not in a multiplayer lobby)

At this point, device A can just simply create a new lobby to get back to a correct state. I can't get device B to create a lobby however even after restarting (probably a different bug regarding creating lobbies on Android devices).

Screenshots or videos showing encountered issue:

osu!lazer version:
2021.815.0-lazer

Logs:
N/A

[peppy]

Please use only one device at a time. osu! doesn't support running on more than one for now.

[peppy]

An eventual fix would see new connections blocked as long as an old one is still active (and within a timeout period).

[frenzibyte]

Seems that would also take over #47, which is tracking the case of having two simultaneous connections from the same user.

[bdach]

Design goals for initial implementation: https://discord.com/channels/188630481301012481/188630652340404224/1170002330616348784
Proof-of-concept version: master...bdach:osu-server-spectator:no-concurrent-connections-poc (broken in multiple respects, does not uphold aforementioned stated goals)