Use class name as class loading lock

On behalf of robert.lyb@alibaba-inc.com

When many thread try to load same class, the thread will stuck on ClassLoader.loadClass.
At current jdk, the stacktrace is:

```
"Thread-4" prio=5 Id=15 BLOCKED on java.lang.Object@b4c966a owned by "Thread-2" Id=13
at java.base@14.0.2/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:612)
- blocked on java.lang.Object@b4c966a
at java.base@14.0.2/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:600)
at java.base@14.0.2/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base@14.0.2/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
at app//Main.test(Main.java:18)
at app//Main$$Lambda$5/0x0000000800b64c40.run(Unknown Source)
at java.base@14.0.2/java.lang.Thread.run(Thread.java:832)

output by Main.java
```

It's not easy to get which class stuck the thread.

If there is a method to get locked Object, and print it in stacktrace, like this:

```
"Thread-2" prio=5 Id=13 BLOCKED on java.lang.String@724af044 owned by "Thread-3" Id=14
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:612)
- blocked on java.lang.String@724af044 val="java.lang.String" --- this String is classname
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:600)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
at app//Main2.test(Main2.java:18)
at app//Main2$$Lambda$38/0x0000000100097440.run(Unknown Source)
at java.base/java.lang.Thread.run(Thread.java:832)


output by Main2.java
```

That is, user will know which class stuck the thread, in this example, the class is java.lang.String. It's helpful for troubleshooting.