code: static String getServerIdFromHandshake(ChannelBuffer handshakeBuffer, ObjectMapper mapper) throws IOException {
          Handshake handshake = getHandshakeFromBuffer(handshakeBuffer, mapper);
          return handshake.getServerId();
      }
docs: Extract the unique id of the Raft server that sent a handshake
  message from its wire representation.
  
  @param handshakeBuffer instance of {@code ChannelBuffer} that contains
  the encoded handshake message
  @param mapper instance of {@code ObjectMapper} used to map handshake fields
  in the encoded message to their corresponding Java representation
  @return unique id of the Raft server that sent the handshake message
  @throws IOException if a valid handshake cannot be read from the {@code handshakeBuffer}