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.

As I understand it, the connection should be terminated rather than remain unhandled in an invalid state.

    public override async Task OnConnectedAsync(ConnectionContext connection)
    {
        ...
        var clientHandler = ClientHandler;
        if (clientHandler != null)
        {
            await clientHandler(adapter).ConfigureAwait(false);
        }
+       else
+       {
+           connection.Abort();
+       }
    }

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

Edited for clarity and to sync up to current master