Prevent Operator star/stop procedure be performed twice

[lburgazzoli]

As today the Operator start/stop methods do not have any barrier to avoid the methods to be invoked more than once and as example, since the introduction of the PR here https://github.com/quarkiverse/quarkus-operator-sdk/blob/main/runtime/src/main/java/io/quarkiverse/operatorsdk/runtime/StartupListener.java, I've observed that my operator did invoke start twice because I also had a startup listener to implement the same functionality.

I have now fixed my code so id does not happen anymore but I feel a barrier on the start/stop methods could be useful in any case.

[lburgazzoli]

@metacosm pr #491 may address this issues