考虑将bulidKey中的判断逻辑下放到KeyConvertor中吗？

[gaoying0805]

public byte[] buildKey(K key) {
    try {
        Object newKey = key;
        if (key instanceof byte[]) {
            newKey = key;
        } else if(key instanceof String){
            newKey = key;
        } else {
            if (config.getKeyConvertor() != null) {
                newKey = config.getKeyConvertor().apply(key);
            }
        }
        return ExternalKeyUtil.buildKeyAfterConvert(newKey, config.getKeyPrefix());
    } catch (IOException e) {
        throw new CacheException(e);
    }
}

考虑后续将key逻辑判断下放到具体的keyConvertor中吗，这样自定义拼接一类的会更加方便，我这面遇到了自定义后缀拼接时，key instanceof String 状态时拼接失效的问题。

[areyouok]

see #749