protected byte decodeProtocolVersion(ByteBuf in) {
        if (in.readableBytes() >= DEFAULT_PROTOCOL_VERSION_LENGTH) {
            return in.readByte();
        }
        return DEFAULT_ILLEGAL_PROTOCOL_VERSION_LENGTH;
    }
decode the protocol version

@param in input byte buf
@return a byte to represent protocol version
src/main/java/com/alipay/remoting/codec/ProtocolCodeBasedDecoder.java