MqttConnectionHandler race condition

[xemed-ahope]

The initial state of MqttConnectionHandler can result in a NullReferenceException in the time between generic host starup and "full" startup. In my test environment this occurs consistently. For a workaround, I explicitly pull and inject the dependencies by calling StartAsync after building and prior to starting the host.

A fix is included with all the other goodies in #2103 .

Per @xljiulang:

If you want to delay starting MqttnetServer, you should fix MqttConnectionHandler as follows:

    public MqttConnectionHandler(IMqttNetLogger logger)
    {
        _logger = logger;
    }
    public override async Task OnConnectedAsync(ConnectionContext connection)
    {
        var clientHandler = ClientHandler;
        if (clientHandler == null)
        {
            connection.Abort();
            _logger.Publish(MqttNetLogLevel.Warning, nameof(MqttConnectionHandler), $"{nameof(MqttServer)} has not been started yet.", null, null);
            return;
        }
        ...
}

Initially called out by @xljiulang in #2102 (comment)

Addendum: Modified to initialize logger by injection, consistent with the full changes in #2103