@Override
  public long getSequenceId() {
    return this.sequenceId;
  }
SequenceId is only available post WAL-assign. Calls before this will get you a
{@link SequenceId#NO_SEQUENCE_ID}. See the comment on FSHLog#append and #getWriteNumber in this
method for more on when this sequenceId comes available.
@return long the new assigned sequence number
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALKeyImpl.java