- [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
for (String n : names) {
                    cacheName(clazz, n);
                    saveInExtensionClass(extensionClasses, clazz, n);
                }
            }
        }
    }
    /**
     * cache name
     */
    private void cacheName(Class<?> clazz, String name) {
        if (!cachedNames.containsKey(clazz)) {
            cachedNames.put(clazz, name);
        }
    }
    /**
     * put clazz in extensionClasses
     */
    private void saveInExtensionClass(Map<String, Class<?>> extensionClasses, Class<?> clazz, String name) {
        Class<?> c = extensionClasses.get(name);
        if (c == null) {
            extensionClasses.put(name, clazz);
        } else if (c != clazz) {
            String duplicateMsg = "Duplicate extension " + type.getName() + " name " + name + " on " + c.getName() + " and " + clazz.getName();
            logger.error(duplicateMsg);
            throw new IllegalStateException(duplicateMsg);
        }
    }
    /**
     * cache Activate class which is annotated with <code>Activate</code>