- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: master
* Operating System version: windows
* Java version: 1.8

In ExtensionLoader, EXTENSION INSTANCES is a ConcurrentHashMap with a default size, and the extension class provided in Dubbo has exceeded 16. 
Start the program, it will expand at least twice. I think it is more appropriate to add an initial size of 64 here. 
Other places where ConcurrentHashMap is used should also pay attention to a suitable initial size
*/
    public List<T> getActivateExtension(URL url, String key) {
        return getActivateExtension(url, key, null);
    }
    /**
     * This is equivalent to {@code getActivateExtension(url, values, null)}
     *
     * @param url    url
     * @param values extension point names
     * @return extension list which are activated
     * @see #getActivateExtension(org.apache.dubbo.common.URL, String[], String)
     */
    public List<T> getActivateExtension(URL url, String[] values) {
        return getActivateExtension(url, values, null);
    }
    /**
     * This is equivalent to {@code getActivateExtension(url, url.getParameter(key).split(","), null)}
     *
     * @param url   url
     * @param key   url parameter key which used to get extension point names
     * @param group group
     * @return extension list which are activated.
     * @see #getActivateExtension(org.apache.dubbo.common.URL, String[], String)
     */
    public List<T> getActivateExtension(URL url, String key, String group) {
        String value = url.getParameter(key);
        return getActivateExtension(url, StringUtils.isEmpty(value) ? null : COMMA_SPLIT_PATTERN.split(value), group);
    }
    /**
     * Get activate extensions.