- [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
instance = createExtension(name);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }
    /**
     * Get the extension by specified name if found, or {@link #getDefaultExtension() returns the default one}
     *
     * @param name the name of extension
     * @return non-null
     */
    public T getOrDefaultExtension(String name) {
        return containsExtension(name)  ? getExtension(name) : getDefaultExtension();
    }
    /**
     * Return default extension, return <code>null</code> if it's not configured.
     */
    public T getDefaultExtension() {
        getExtensionClasses();
        if (StringUtils.isBlank(cachedDefaultName) || "true".equals(cachedDefaultName)) {
            return null;
        }
        return getExtension(cachedDefaultName);
    }
    public boolean hasExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }