Support LRU evicting mechanism for origin statistic map in StatisticNode

## Issue Description

Type: feature request

### Describe what happened (or what feature you want)
我们的业务场景，希望能能够按照调用方的IP来限流，Sentinel 目前支持按照调用源来限流，但是没有淘汰策略，业务真正使用起来，假如IP数量比较多，风险有点大，所以希望能够支持LRU，业务使用起来更稳定。

其实热点参数限流，目前是支持LRU的，但是为什么不能用热点参数来限流呢？
热点参数限流，一般情况下，需要把限流的源绑定到参数上，这样才可以使用。但是实际的业务场景很复杂，例如上游服务不愿意改动，所以用热点参数限流可能做起来就不是那么的方便。所以才需要能够按照调用源来限流。例如按IP，这样只需要获取到上游服务的IP就行，并且IP过多的情况下，还支持LRU，就比较稳妥了。

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

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### Tell us your environment

### Anything else we need to know?

Start at StatisticNode and inspect how the origin statistic map is maintained. Compare it with the existing LRU support for hot-parameter limiting, then define the bounded eviction behavior for origin-based limiting and verify that callers can use it for high-cardinality sources such as IP addresses.

Written by the indexing model from the issue text.