code: public static Field getCasVersionColumn(Class<?> clazz) throws CasVersionNotMatchException {
  		List<Field> fields = getColumns(clazz);
  		Field casVersionField = null;
  		for(Field field : fields) {
  			Column column = field.getAnnotation(Column.class);
  			if(column.casVersion()) {
  				if(casVersionField != null) {
  					throw new CasVersionNotMatchException("there are more than one casVersion column in a DO class");
  				}
  				casVersionField = field;
  			}
  		}
  		return casVersionField;
  	}
docs: 获得一个DO类注解的casVersion字段
  @param clazz
  @return 当没有column字段时返回null
  @throws CasVersionNotMatchException 当有2个及2个以上的casVersion字段时抛出该异常