Add option to run the aeron-client-conductor thread as a background thread.

[Tomius]

The AgentRunner on this line starts a foreground thread for the aeron client conductor.

So if I use this and the main thread of my application crashes in a way that is not able to Dispose the AeronClient, then this thread will keep the application alive indefinitely. I find this behavior very undesirable.

Would it be possible to add an option to start it as a background thread?