Re-read the leaf element when atomic CAS fails due to a race with
another thread that has locked the leaf element, since
atomic_compare_exchange_strong_p() overwrites the expected value with
the actual value on failure. This regression was introduced by
0ee0e0c155a05d0d028a9972ad86b9eaac4ccabd (Implement compact rtree leaf
element representation.).
This resolves#798.