code: OutboundTcpConnection getConnection(MessageOut msg)
      {
          Stage stage = msg.getStage();
          return stage == Stage.REQUEST_RESPONSE || stage == Stage.INTERNAL_RESPONSE || stage == Stage.GOSSIP
                 ? ackCon
                 : cmdCon;
      }
docs: returns the appropriate connection based on message type.
  returns null if a connection could not be established.