void freeBucketEntry(BucketEntry bucketEntry) {
    bucketAllocator.freeBlock(bucketEntry.offset(), bucketEntry.getLength());
    realCacheSize.add(-1 * bucketEntry.getLength());
  }
Free the {{@link BucketEntry} actually,which could only be invoked when the
{@link BucketEntry#refCnt} becoming 0.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java