@Override
    public String toString() {
      return CellUtil.getCellKeyAsString(toCell());
    }
Copy the state from the next one into this instance (the previous state placeholder). Used to
save the previous state when we are advancing the seeker to the next key/value.
hbase-common/src/main/java/org/apache/hadoop/hbase/io/encoding/RowIndexSeekerV1.java