fix ConcurrentModificationException

[jubatusd]

I noticed that the use of clearableRefs in the method clearAllReferences method is incorrect.
The type of clearableRefs is synchronizedList. Although the safeList is thread-safe, its iterator is not.
According to Javadocs :

It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream

I think this is the root cause of the occurrence of ConcurrentModificationException.

To analyze more specifically, why is there a very small chance of a ConcurrentModificationException occurring

When the use of heap memory exceeds the limit, notificationListener.notifyHeapAboveThreshold() will be called. It runs the clearing of references asynchronously there, which involves modifying clearableRefs.

Is it possible that when the clearAllReferences() method is called, some of the threads initiated by the notificationListener.notifyHeapAboveThreshold() method have not finished executing. This leads to the use of clearableRefs.spliterator() to generate the iterator of clearableRefs for traversal, and clearableRefs is modified by other threads again.

Resolves #424

[mpollmeier]

makes sense, thank you!

[mpollmeier]

I'll push the upgrade through cpg and joern