Don't install a shutdown hook by default

[lburgazzoli]

When the Operator class is instantiated, it automatically registers a shutdown hook:

Runtime.getRuntime().addShutdownHook(new Thread(this::close))

As the startup of the operator is responsibility of the operator developer, I think the shutdown should be left to the user otherwise there may be some conflict and undefined behavior because there may be the need to perform some logic before the operator shuts down.

I'm proposing to make this behavior explicit, like

operator.installShutdownHook();