[server] Set socket to null in finishClosingSocket
[server] Set socket to null in finishClosingSocket

This means `onClose` won't get called for it, but is necessary for the `this.reopenConnectionAfterClosing` logic to work correctly. We'll just do everything we would do in `onClose` in `finishClosingSocket` as well.

Also, moved the socket code to handle `ServerRequest`s to the corresponding two async functions.

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