delete method
Deletes a value by key.
Implementation
Future<void> delete(String key) async {
final r = await _send(opDelete, key, Uint8List(0));
if (r.isDenied) throw WsDeniedException('DEL: ${utf8.decode(r.data)}');
if (r.isError) throw WsException('DEL: ${utf8.decode(r.data)}');
}