Feature request : support Disconnect event for room matched by "next?"

[Fcornaire]

Hello, i noticed that a peer matched by a next? never receives disconnected event.
From what I understand in the codebase, this happens because the room is purged once all peers have joined (likely to make space for the next clients)., thus making disconnected peers never able to notify others in that room.

This PR addresses the issue by saving the full list of room peers before purging it, allowing those events to be sent later.

This may not be the best way to handle this, let me know if any changes are needed!