- [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
if (extensionLoaderClassLoaderFirst) {
                ClassLoader extensionLoaderClassLoader = ExtensionLoader.class.getClassLoader();
                if (ClassLoader.getSystemClassLoader() != extensionLoaderClassLoader) {
                    urls = extensionLoaderClassLoader.getResources(fileName);
                }
            }
            
            if(urls == null || !urls.hasMoreElements()) {
                if (classLoader != null) {
                    urls = classLoader.getResources(fileName);
                } else {
                    urls = ClassLoader.getSystemResources(fileName);
                }
            }
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    java.net.URL resourceURL = urls.nextElement();
                    loadResource(extensionClasses, classLoader, resourceURL, excludedPackages);
                }
            }
        } catch (Throwable t) {
            logger.error("Exception occurred when loading extension class (interface: " +
                    type + ", description file: " + fileName + ").", t);
        }
    }
    private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader,
                              java.net.URL resourceURL, String... excludedPackages) {
        try {
            try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), StandardCharsets.UTF_8))) {
                String line;