We currently clean weak roots only when weak-references are processed in Traversal GC. This is wrong, because it should be independent whether or not it's doing reference-processing.
Also, (weak) roots need to be updated and cleaned before classes get unloaded (currently), because this is how class-unloaded detects unreachable classes.
With all this, we no longer need to run class-unloading and reference-processing every cycle, which covered up those bugs.