Dying objects are published from users of CollectedHeap::object_iterate

Caution needs to be taken when CollectedHeap::object_iterate is used. It may:
- Return non-reachable objects - for example, when G1 walks the prev bitmap
- Return the Reference.referent without an appropriate resurrection barrier
- Return an object loaded without a "strong"/"marking" barrier

These objects should not escape the confines of the active safepoint that calls object_iterate. If they do we run the risk of various crashes when the objects get garbage collected.

Unfortunately, there are a few closures that do this. The once we have identified are:
- HeapInspection::find_instances_at_safepoint
- JVMTI object tagging

There might be some merit to solving this by altering the object_iterate API, but for now we'd like to fix the two problematic areas listed above. The suggestion is to introduce a CollectedHeap::keep_alive function that notifies the GC that the object should be kept alive the current GC cycle (if active).