Re-think how we can handle collaboratively closing expired positions

[da-kami]

At the moment we take a shortcut and just collaboratively close the contract with a BitMEX price.

This is not correct, i.e. the coordinator should not take any risk here. Instead, it should take a rate it would get from the orderbook and collab close against it.
Differently said, the coordinator should pass on the position to another trader which results in another match.

Originally mentioned here: #967 (comment)

[github-actions]

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