Add more spotbugs checks and fix existing issues

[aneveux]

Hello folks 👋

I'm currently following the Improve a Plugin documentation for
Jenkins plugins modernization, and noticed we could add more spotbugs checks for this plugin.

Adding additional Spotbugs checks did raise quite a bunch of warnings which I tried to fix in that PR.
For some of them, I was a bit uncertain about either the expected outcome, or the risk of changing things, so I instead ignored the warning, as it is the current state of the source code.

So here's a PR doing that. I checked that everything is running fine with mvn verify.

Thanks for your review!

Let me know if there is anything you would like me to take care of in that PR. If you believe it is not worth for the project, feel free to close it.

Testing done

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6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[Vlatombe]

Looks good though some of the spotbugs check seem overkill.

[Vlatombe]

Thank you for the PR @aneveux !

[Vlatombe]

Test failures don't look like flakes (at least look at the ones under jdk11, they are independent from a real k8s cluster)

[aneveux]

Test failures are now fixed. I thought I could put that kc.connect() in a try-with-resource block, but that is actually what is failing the test. I believe the mutation has some effect elsewhere in the code that is necessary for the current behavior.

[aneveux]

New test failures, I am looking at it.

[aneveux]

@Vlatombe I finally got some time to fix the remaining issues here and merge with master.

LMK if that PR is fine for you ❤️

[jglick]

I found a problem here: if this method is called from another thread used synchronously during launch (JnlpConnectionState.fireAfterChannel) while launch(…) is running, you can get a deadlock.