@Override
  public Optional<Map<String, Pair<String, Long>>> getFullyCachedFiles() {
    return Optional.of(fullyCachedFiles);
  }
Defined an {@link Consumer} here, because once the removed entry release its reference count,
then it's ByteBuffers may be recycled and accessing it outside this method will be thrown an
exception. the consumer will access entry to remove before release its reference count.
Notice, don't change its reference count in the {@link Consumer}
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java