Use array instead of set for storing callbacks

lib/www/server/ws/db.js

@@ -33,11 +33,11 @@ async function listen (addChannels, callback) {
 	for (const channel of addChannels) {
 		if (!(channel in channels)) {
 			await client.query("LISTEN "+channel);
-			channels[channel] = new Set();
+			channels[channel] = [];
 			console.log("Listening to ", channel);
 		}
 
-		channels[channel].add(callback);
+		channels[channel].push(callback);
 	}
 }