If this happens often in practice, rework flow so that when matchmaking room is full and all connections are alive

[github-actions[bot]]

then have the final connection hand-off be concurrently run without needing to be awaited.

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https://github.com/fishfolk/bones/blob/2f4e7ac48b8c3a6e2617fef5f7e98bf984281943/other_crates/bones_matchmaker/src/matchmaking.rs#L112-L119

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This issue was generated by todo-issue based on a TODO comment in 2f4e7ac48b8c3a6e2617fef5f7e98bf984281943.