- [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
private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
    private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<>();
    private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<>();
    private final Class<?> type;
    private final ExtensionFactory objectFactory;
    private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<>();
    private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>();
    private final Map<String, Object> cachedActivates = new ConcurrentHashMap<>();
    private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
    private final Holder<Object> cachedAdaptiveInstance = new Holder<>();
    private volatile Class<?> cachedAdaptiveClass = null;
    private String cachedDefaultName;
    private volatile Throwable createAdaptiveInstanceError;
    private Set<Class<?>> cachedWrapperClasses;
    private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<>();
    private static LoadingStrategy DUBBO_INTERNAL_STRATEGY =  () -> DUBBO_INTERNAL_DIRECTORY;
    private static LoadingStrategy DUBBO_STRATEGY = () -> DUBBO_DIRECTORY;
    private static LoadingStrategy SERVICES_STRATEGY = () -> SERVICES_DIRECTORY;
    private static LoadingStrategy[] strategies = new LoadingStrategy[] { DUBBO_INTERNAL_STRATEGY, DUBBO_STRATEGY, SERVICES_STRATEGY };
    public static void setLoadingStrategies(LoadingStrategy... strategies) {
        ExtensionLoader.strategies = strategies;
    }
    private ExtensionLoader(Class<?> type) {
        this.type = type;
        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
    }
    private static <T> boolean withExtensionAnnotation(Class<T> type) {
        return type.isAnnotationPresent(SPI.class);
    }
    @SuppressWarnings("unchecked")