Don't fail on initial dial when autoreconnect is true

[wipsel]

When an application is started and the opc-ua server is not online at the start of the application. The client obviously cannot connect. In the current situation the Connect method fails with an error.
But in some use cases it would be useful to create the client anyway add monitored items etc and just connect whenever the server is online again.

Maybe a method could be added TryConnect(ctx context.Context) that does an initial dial and if it fails it starts the reconnect loop. Or we could update the existing Connect method to adapt this behavior when AutoReconnect is true.

Let me know if this is useful or if there is another way to achieve the behavior I am describing. If this is something useful for the project let me know and I could look into fixing this.

[Scaredon]

@wipsel
+1
this would help a lot to make telegraf - opcua more usable.