docs: private void sendFirstFragResponse()
      {
          if (ELASTICLOG.isDebugEnabled()) {
              ELASTICLOG.debug("P" + m_partitionId + " sending first fragment response to coordinator " +
                      CoreUtils.hsIdToString(m_coordinatorHsId));
          }
          RejoinMessage msg = new RejoinMessage(m_mailbox.getHSId(),
                  RejoinMessage.Type.FIRST_FRAGMENT_RECEIVED);
          m_mailbox.send(m_coordinatorHsId, msg);
          m_firstFragResponseSent = true;
      }