code: void sendProposedEpoch() throws IOException {
      Message message = MessageBuilder
                        .buildProposedEpoch(persistence.getProposedEpoch(),
                                            persistence.getAckEpoch(),
                                            persistence.getLastSeenConfig(),
                                            getSyncTimeoutMs());
      if (LOG.isDebugEnabled()) {
        LOG.debug("Sends {} to leader {}", TextFormat.shortDebugString(message),
                  this.electedLeader);
      }
      sendMessage(this.electedLeader, message);
    }
docs: Sends CEPOCH message to its prospective leader.
  @throws IOException in case of IO failure.