unsubscribe method
Implementation
Future<void> unsubscribe(
String destination,
) async {
final id = _subscriptionsMap['destination'];
if (id == null) {
throw Exception('No subscription found for destination: $destination');
}
_tcpClient.add(ClientFrame.unsubscribe(id).toBytes());
_subscriptionsMap.remove(destination);
_handlers.remove(destination);
}