Fix data race of Token result Pool

[louyuting]

Describe what this PR does / why we need it

==================
WARNING: DATA RACE
Read at 0x00c0000a0a00 by goroutine 11:
  github.com/alibaba/sentinel-golang/core/base.(*TokenResult).IsBlocked()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/result.go:58 +0x351
  github.com/alibaba/sentinel-golang/core/stat.(*StatisticSlot).OnCompleted()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/stat/stat_slot.go:32 +0x40
  github.com/alibaba/sentinel-golang/core/base.(*SlotChain).exit()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/slot_chain.go:181 +0x9f
  github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit.func1()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/entry.go:64 +0xa3
  sync.(*Once).doSlow()
      /usr/local/go/src/sync/once.go:66 +0x103
  sync.(*Once).Do()
      /usr/local/go/src/sync/once.go:57 +0x68
  github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/entry.go:62 +0x144
  main.main.func2()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/test/cb.go:67 +0x136

Previous write at 0x00c0000a0a00 by goroutine 10:
  github.com/alibaba/sentinel-golang/core/base.RefurbishTokenResult()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/result.go:118 +0x40a
  github.com/alibaba/sentinel-golang/core/base.(*SlotChain).Entry()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/core/base/slot_chain.go:145 +0x401
  github.com/alibaba/sentinel-golang/api.entry()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/api.go:114 +0x4c1
  github.com/alibaba/sentinel-golang/api.Entry()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/api.go:86 +0x19a
  main.main.func1()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/test/cb.go:47 +0x58

Goroutine 11 (running) created at:
  main.main()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/test/cb.go:58 +0x439

Goroutine 10 (running) created at:
  main.main()
      /Users/sczyh30/temp/to-review/sentinel-go-ximu/api/test/cb.go:45 +0x421
==================

Root cause is here:
Refer to: https://github.com/alibaba/sentinel-golang/commit/975debd7075d071473fcbcd17c3dbc3bd201a2da#diff-604f4192deea5adf9c159e5f88f64630R93

defer func() {
	if r != nil {
		base.RefurbishTokenResult(r)
	}
}()

r is the same Pointer with context.Output.LastResult. Sentinel refurbish the r. But When user call Exit() by manually, will call context.Output.LastResult(the refurbished TokenResult might be got by other goroutine), it causes DATA RACE.

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

Special notes for reviews

[sczyh30]

Why a copy is needed here?

[louyuting]

It's not needed, maybe return r is enough

[sczyh30]

Check result nil here? (in case previous customized slots cleared the result)

[louyuting]

make sense

[codecov-commenter]

Codecov Report

Merging #149 into master will decrease coverage by 0.76%.
The diff coverage is 33.12%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   43.48%   42.72%   -0.77%     
==========================================
  Files          67       67              
  Lines        2863     2926      +63     
==========================================
+ Hits         1245     1250       +5     
- Misses       1479     1537      +58     
  Partials      139      139              

Impacted Files	Coverage Δ
core/base/block_error.go	10.00% <0.00%> (-18.58%)	⬇️
core/circuit_breaker/slot.go	0.00% <0.00%> (ø)
core/flow/slot.go	0.00% <0.00%> (ø)
core/flow/tc_default.go	0.00% <0.00%> (ø)
core/stat/stat_slot.go	0.00% <0.00%> (ø)
core/base/context.go	25.00% <23.07%> (-27.39%)	⬇️
core/base/result.go	29.35% <25.67%> (-8.98%)	⬇️
core/system/slot.go	56.60% <37.50%> (-3.82%)	⬇️
core/flow/tc_throttling.go	66.66% <50.00%> (ø)
core/circuit_breaker/circuit_breaker.go	46.21% <57.14%> (ø)
... and 3 more

---

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[sczyh30]

LGTM

[sczyh30]

Thanks!