- [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
return (T) instance;
    }
    private IllegalStateException findException(String name) {
        for (Map.Entry<String, IllegalStateException> entry : exceptions.entrySet()) {
            if (entry.getKey().toLowerCase().contains(name.toLowerCase())) {
                return entry.getValue();
            }
        }
        StringBuilder buf = new StringBuilder("No such extension " + type.getName() + " by name " + name);
        int i = 1;
        for (Map.Entry<String, IllegalStateException> entry : exceptions.entrySet()) {
            if (i == 1) {
                buf.append(", possible causes: ");
            }
            buf.append("\r\n(");
            buf.append(i++);
            buf.append(") ");
            buf.append(entry.getKey());
            buf.append(":\r\n");
            buf.append(StringUtils.toString(entry.getValue()));
        }
        return new IllegalStateException(buf.toString());
    }
    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {