release method
Destroy and stop event to the client with related channels.
Implementation
Future<void> release() async {
await _eventSubscription
?.cancel()
.then((value) => _eventSubscription = null);
await Future.forEach<AgoraRtmChannel>(
_channels.values, (element) => element.release());
await _callNative("release", null);
_clients.removeWhere((int key, _) => key == _clientIndex);
}