- [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
* @param clazz extension class
     * @throws IllegalStateException when extension with the same name has already been registered.
     */
    public void addExtension(String name, Class<?> clazz) {
        getExtensionClasses(); 
        if (!type.isAssignableFrom(clazz)) {
            throw new IllegalStateException("Input type " +
                    clazz + " doesn't implement the Extension " + type);
        }
        if (clazz.isInterface()) {
            throw new IllegalStateException("Input type " +
                    clazz + " can't be interface!");
        }
        if (!clazz.isAnnotationPresent(Adaptive.class)) {
            if (StringUtils.isBlank(name)) {
                throw new IllegalStateException("Extension name is blank (Extension " + type + ")!");
            }
            if (cachedClasses.get().containsKey(name)) {
                throw new IllegalStateException("Extension name " +
                        name + " already exists (Extension " + type + ")!");
            }
            cachedNames.put(clazz, name);
            cachedClasses.get().put(name, clazz);
        } else {
            if (cachedAdaptiveClass != null) {
                throw new IllegalStateException("Adaptive Extension already exists (Extension " + type + ")!");
            }
            cachedAdaptiveClass = clazz;
        }
    }