OopStorage usage rules should be documented and checked

JDK-8251336 showed that recent code changes to use OopStorage were flawed because the release operation could be carried out whilst a safepoint were active.

As [~rkennke] noted:

In general, non-GC threads should not mess with GC roots (e.g.
OopStorage) during safepoints. I don't think that this plays well with
any GC. Not sure if it can be automatically detected, it sure would be
nice if that would fire an assert or so.