cup bufferpool,  add extendibility 

**Is your feature request related to a problem? Please describe.**
简洁描述下新feature的内容

cup bufferpool cannot be extended.  Add extendibility to it.

**Describe the solution you'd like**
描述下你想要什么样的解决方案

**Describe alternatives you've considered**
是否有其他替代方案

**Additional context**
可以辅助其他人理解你的信息
