improve `find_unique_connection_index`

[piot]

• free list or similar to find a free ID
• Result<> instead of panic if no free ID is found

room-session/crates/session/src/lib.rs

Lines 280 to 291 in 27e5932

	fn find_unique_connection_index(&self) -> ConnectionIndex {
	let mut candidate = self.id;
	while self.connections.contains_key(&candidate) {
	candidate.next();
	if candidate == self.id {
	panic!("No unique connection index available");
	}
	}
	candidate
	}