Investigate sending shared data through websockets

[adryzz]

This server is supposed to have a lot of open websockets, and sending messages to thousands if not tens of thousands of clients through a websocket is an expensive operation, especially if a clone is involved.

axum uses tungstenite-rs as its default websocket implementation, which currently requires cloning data (no Arc<T> either). This is being tracked over at snapview/tungstenite-rs#96.

This would really speed things up, as there isn't THAT much more we can do to improve performance other than there, as shown by the flamegraph.