default Optional<Map<String, Pair<String, Long>>> getFullyCachedFiles() {
    return Optional.empty();
  }
Returns an Optional containing the map of files that have been fully cached (all its blocks are
present in the cache. This method may not be overridden by all implementing classes. In such
cases, the returned Optional will be empty.
@return empty optional if this method is not supported, otherwise the returned optional
        contains a map of all files that have been fully cached.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java