Feature： new slot chain

[AlexStocks]

Describe what this PR does / why we need it

Sentinel 包针对某个 Resource 无法确知其使用了那个组件，在运行时会针对某个 Resource 的 EntryContext 依次执行所有的组件的 Rule。本pr的目的就是减少下面函数 SlotChain.Entry() 中执行 RuleCheckSlot.Check() 执行次数

Describe how you did it

最后优化的效果的集合代码如下：

	sc, ok := rsSlotChain[rsName]
	if !ok {
		sc = base.NewSlotChain()
		sc.AddStatPrepareSlotLast(stat.DefaultResourceNodePrepareSlot)

		sc.AddStatSlotLast(stat.DefaultSlot)
		sc.AddStatSlotLast(log.DefaultSlot)
	}

	switch slotName {
	case system.RuleCheckSlotName:
		if !validateRuleCheckSlot(sc, system.DefaultAdaptiveSlot) {
			sc.AddRuleCheckSlotLast(system.DefaultAdaptiveSlot)
		}

	case flow.RuleCheckSlotName:
		if !validateRuleCheckSlot(sc, flow.DefaultSlot) {
			sc.AddRuleCheckSlotLast(flow.DefaultSlot)
			sc.AddStatSlotLast(flow.DefaultStandaloneStatSlot)
		}

	case isolation.RuleCheckSlotName:
		if !validateRuleCheckSlot(sc, isolation.DefaultSlot) {
			sc.AddRuleCheckSlotLast(isolation.DefaultSlot)
		}

	case circuitbreaker.RuleCheckSlotName:
		if !validateRuleCheckSlot(sc, circuitbreaker.DefaultSlot) {
			sc.AddRuleCheckSlotLast(circuitbreaker.DefaultSlot)
			sc.AddStatSlotLast(circuitbreaker.DefaultMetricStatSlot)
		}

	case hotspot.RuleCheckSlotName:
		if validateRuleCheckSlot(sc, hotspot.DefaultSlot) {
			sc.AddRuleCheckSlotLast(hotspot.DefaultSlot)
			sc.AddStatSlotLast(hotspot.DefaultConcurrencyStatSlot)
		}
	}

[CLAassistant]

All committers have signed the CLA.

[louyuting]

Otherwise, could you please resolve the conflicts(rebase the newest code from alibaba/sentinel-golang)

[codecov-io]

Codecov Report

Merging #264 into master will increase coverage by 0.14%.
The diff coverage is 82.60%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   51.09%   51.23%   +0.14%     
==========================================
  Files          79       79              
  Lines        4026     4048      +22     
==========================================
+ Hits         2057     2074      +17     
- Misses       1679     1684       +5     
  Partials      290      290              

Impacted Files	Coverage Δ
api/api.go	19.44% <0.00%> (-0.85%)	⬇️
core/isolation/slot.go	0.00% <0.00%> (ø)
core/base/slot_chain.go	92.95% <78.57%> (-4.06%)	⬇️
api/slot_chain.go	93.33% <100.00%> (+12.08%)	⬆️
core/circuitbreaker/rule_manager.go	71.83% <100.00%> (+0.32%)	⬆️
core/circuitbreaker/slot.go	100.00% <100.00%> (ø)
core/circuitbreaker/stat_slot.go	14.28% <100.00%> (+14.28%)	⬆️
core/flow/rule_manager.go	61.36% <100.00%> (+0.29%)	⬆️
core/flow/slot.go	44.82% <100.00%> (+1.97%)	⬆️
core/flow/standalone_stat_slot.go	71.42% <100.00%> (+4.76%)	⬆️
... and 6 more

---

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[louyuting]

Thanks for your nice work!