- [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
import java.util.concurrent.ConcurrentMap;
import java.util.regex.Pattern;
import static java.util.Collections.sort;
import static org.apache.dubbo.common.constants.CommonConstants.COMMA_SPLIT_PATTERN;
import static org.apache.dubbo.common.constants.CommonConstants.DEFAULT_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.REMOVE_VALUE_PREFIX;
/**
 *
 * {@link org.apache.dubbo.rpc.model.ApplicationModel}, {@code DubboBootstrap} and this class are
 * at present designed to be singleton or static (by itself totally static or uses some static fields).
 * So the instances returned from them are of process or classloader scope. If you want to support
 * multiple dubbo servers in a single process, you may need to refactor these three classes.
 *
 * Load dubbo extensions
 * <ul>
 * <li>auto inject dependency extension </li>
 * <li>auto wrap extension in wrapper </li>
 * <li>default extension is an adaptive instance</li>
 * </ul>
 *
 * @see <a href="http:java.sun.com/j2se/1.5.0/docs/guide/jar/jar.html#Service%20Provider">Service Provider in Java 5</a>
 * @see org.apache.dubbo.common.extension.SPI
 * @see org.apache.dubbo.common.extension.Adaptive
 * @see org.apache.dubbo.common.extension.Activate
 */
public class ExtensionLoader<T> {
    private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);
    private static final String SERVICES_DIRECTORY = "META-INF/services/";
    private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";