Add support for client reconnection

[bdach]

Disregarding all discussion above, a first point to address is the ability for a user to reconnect to osu-server-spectator. Regardless of whether we move responsibilities around, we need to ensure that a dropped websocket connection is not the end of the world for multiplayer, spectator, and (currently) replay storage.

Once we have this in place, we can decide whether responsibilities need to be shuffled.

Originally posted by @peppy in ppy/osu#24609 (comment)

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The key part for the issue above is spectating flows, as they are the ones that implicitly participate in submission to upload replays. This will likely require a reconciliation flow which ensures that the client resends any frames the server might not have received.

The rest is also important for other reasons, but strictly speaking not for the aforementioned issue.

[peppy]

Moving out of project.