deleteMessage method
Deletes messageId on the backend.
Implementation
@override
Future<DeliveryStatus> deleteMessage(String messageId) async {
if (!_connected) return DeliveryStatus.fail('disconnected');
await _delay();
_store.removeWhere((x) => x.id == messageId || x.remoteId == messageId);
_events.add(MessageDeletedEvent(messageId));
return DeliveryStatus.ok(messageId);
}