Fix active connectors leaking after refactor

The activeConnectors mapping is from session id to connector, but the
master refactor in 67219ed accidentally changed it to try deleting the
entry by the connector reference instead.  Possibly mistaking it for a
Set instead of a Map.

packages/master/src/WsServer.js

@@ -258,7 +258,7 @@ ${err.stack}`
 		let connector = new WsServerConnector(sessionId, sessionTimeout, heartbeatInterval);
 		this.activeConnectors.set(sessionId, connector);
 		connector.on("close", () => {
-			this.activeConnectors.delete(connector);
+			this.activeConnectors.delete(sessionId);
 		});
 
 		let additionalReadyData = {};