Replace custom GUID generator with UUID v5

[cetteup]

The current GUID generator creates UUID-looking ids that aren't actually UUIDs. It also only works for games using {ip}:{port} to identify servers. To simplify things and gain more flexibility, switching to Python's built-it UUID v5 implementation seems like a good option. Using a game-specific namespace/prefix would even ensure that ids are unique across games.

Note: This will be a breaking change, as old GUIDs will not be matched with the new ones. I might be worth implementing a simple one-time migration.