@Override
    public void clear() {
      blockKeys.clear();
      blockOffsets.clear();
      onDiskDataSizes.clear();
      secondaryIndexOffsetMarks.clear();
      numSubEntriesAt.clear();
      curTotalNonRootEntrySize = 0;
      curTotalRootSize = 0;
    }
The same as {@link #add(byte[], long, int, long)} but does not take the key/value into
account. Used for single-level indexes.
@see #add(byte[], long, int, long)
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlockIndex.java