public int getPerEntryLRUOverhead() {
    BucketRegion br = (localBucket2RegionMap.values().iterator().next());
    return br.getRegionMap().getEntryOverhead();
  }
Test hook to return the per entry overhead for a bucket region. PRECONDITION: a bucket must
exist and be using LRU.

@since GemFire 6.1.2.9
geode-core/src/main/java/org/apache/geode/internal/cache/PartitionedRegionDataStore.java