proxy instance cause a PermSpace OOM

[libo3603]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.8
• Operating System version: window 7
• Java version: 1.8.0_261

Steps to reproduce this issue

1. User Proxy to create new instance of any interface
2. System full GC
3. Check PermSpace unuse size

Pls. provide [GitHub address] to reproduce this issue.

Expected Result

PermSpace used size will no increase
永久区使用空间不会明显增长

What do you expected from the above steps？

Actual Result

PermSpace used size will increase after very Full GC, which cause a OutOfMemoryError [PermSpace]
永久区使用空间在每次full GC之后都会有所增加，直到永久区内存溢出。

If there is an exception, please attach the exception trace:
java.lang.RuntimeException: by java.lang.OutOfMemoryError: PermGen space

Just put your stack trace here!

java.lang.RuntimeException: by java.lang.OutOfMemoryError: PermGen space

[libo3603]

因为 org.apache.dubbo.common.bytecode.Proxy 中使用的Proxy对象缓存导致。
PROXY_CACHE_MAP 缓存的Proxy实例，使用WeakReference，full GC 后会释放该Proxy实例
再次申请对象实例时，Proxy会重新创建Proxy的Class对象，最终导致PermGen space内存溢出。
应该修改为缓存该Proxy Class，而不是 Proxy 对象实例。