@Override
  public boolean release() {
    return refCnt.release();
  }
We've three cases to release refCnt now: <br>
1. BucketCache#evictBlock, it will release the backingMap's reference by force because we're
closing file or clear the bucket cache or some corruption happen. when all rpc references gone,
then free the area in bucketAllocator. <br>
2. BucketCache#returnBlock . when rpc shipped, we'll release the block, only when backingMap
also release its refCnt (case.1 will do this) and no other rpc reference, then it will free the
area in bucketAllocator. <br>
3.evict those block without any rpc reference if cache size exceeded. we'll only free those
blocks with zero rpc reference count.
@return true to indicate we've decreased to zero and do the de-allocation.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketEntry.java