public Task <ServerMessage> PrepareClientMessage(FlatBufferBuilder aBuilder)
{
var id = NextMsgId;
// The client always increments the IDs on outgoing messages
ClientMessage.AddId(aBuilder, id);
var promise = new TaskCompletionSource <ServerMessage>();
_waitingMsgs.TryAdd(id, promise);
Console.WriteLine($"Sending a client message with Id {id}");
return(promise.Task);
}