rumqttc: change connection options after creating client instance

[xiaocq2001]

Expected Behavior

We do not appear to currently have a way to connect with clean_session = True on the initial connect, and then change to using clean_start = False on successive reconnects (i.e. we want to start a clean session, and then if the connection is lost, try and re-establish the same session).

Current Behavior

The options are saved in EventLoop instance, there is no way to modify it after the client and event loop is created. Re-connection uses the options in EventLoop and there is no way to modify them.