default Optional<Boolean> blockFitsIntoTheCache(HFileBlock block) {
    return Optional.empty();
  }
Checks whether there's enough space left in the cache to accommodate the passed block. This
method may not be overridden by all implementing classes. In such cases, the returned Optional
will be empty. For subclasses implementing this logic, the returned Optional would contain the
boolean value reflecting if the passed block fits into the remaining cache space available.
@param block the block we want to check if fits into the cache.
@return empty optional if this method is not supported, otherwise the returned optional
        contains the boolean value informing if the block fits into the cache available space.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java