close method
close connection to NATS server unsub to server but still keep subscription list at client
Implementation
Future close() async {
_setStatus(Status.closed);
_backendSubs.forEach((_, s) => s = false);
_inboxs.clear();
await _wsChannel?.sink.close();
_wsChannel = null;
await _secureSocket?.close();
_secureSocket = null;
await _tcpSocket?.close();
_tcpSocket = null;
_buffer = [];
_clientStatus = _ClientStatus.closed;
}