ConcurrentModificationException in DefaultClusteringRender

[newmanw]

I am getting a ConcurrentModificationException in the DefaultClusteringRender. This happens on line 383 (call to markersToRemove.removeAll):

        // Wait for all markers to be added.
        markerModifier.waitUntilFree();

        // Don't remove any markers that were just added. This is basically anything that had
        // a hit in the MarkerCache.
        markersToRemove.removeAll(newMarkers);

Not 100% sure what the root of this problem is. I.E. should that set be a 'ConcurrentSet' or am I doing something wrong. My guess as to what is happening is that I am performing add and deletes that could be happening on different threads. In which case I would think this is an issue with the Set in DefaultClusteringRender.

Thoughts?

[newmanw]

sorry. This is a dup of #56.