Address FindBugs issues and disable some detectors

This commit fixes enough FindBugs issues and disables enough detectors for `mvn findbugs:check` to pass.
> these are two different mechanisms.

Indeed. But from what I can see, the fact that it's a ConcurrentMap here doesn't prevent the race condition ;-( If we want to use ConcurrentMap properly here, I guess we should try to rewrite this bit to use putIfAbsent?

ap