I have fixed all remaining warnings, but the ones on QuorumCnxManager. I want to check if they are being fixed in one of the current pending patches that Vishal has been working on. It makes more sense to fix them separately.
For Follower, Observer, and Learner, I have removed the synchronization blocks around pendingRevalidations. It seems ok to me to remove them, and this is what findbugs was apparently complaining about.
For AuthFLE, I have fixed synchronization blocks in a different way, by simply replacing the objects we were synchronizing on. In the case of acksqueue, I have replaced the data structure we were using from LinkedBlockQueue to HashSet. It made sense to me to make the change given the way were using acksqueue and fixes the warning.