Fix concurrent map iteration & write issue

[Blaxon]

The issue was introduced in func (l *fileStore) Remove(addr string) error
https://github.com/qiniu/goc/blob/master/pkg/cover/store.go#L107

when we set the memory server list back to file.
the list l.memoryStore.GetAll() -> services passed into func (l *fileStore) Set(services map[string][]string) error
in L175 we sign the services directly to l.memoryStore.servicesMap, without copying it.

while in L186 we are still using this map to do the iteration without lock. (by right should use memoryStore.mu.

The issue will happen when there is another goroutine tries to remove the server from memoryStore. Map will clash.

[Blaxon]

Solve issue: #324

[CarlJi]

/assign @CarlJi

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[Blaxon]

Hi @CarlJi , I just made a new commit

1. to fix the fileStore.Remove data inconsistency issue,
2. and update the unit test to be verifiable.
   Pls help check, Thanks!

[codecov]

Codecov Report

Merging #329 (b6cebb2) into master (5868a3e) will decrease coverage by 0.25%.
The diff coverage is 76.19%.

❗ Current head b6cebb2 differs from pull request most recent head 00142a6. Consider uploading reports for the commit 00142a6 to get more accurate results

Additional details and impacted files

@@ Coverage Diff @@ ## master #329 +/- ## ========================================== - Coverage 70.76% 70.51% -0.25%  ========================================== Files 34 34 Lines 2035 2042 +7 ========================================== Hits 1440 1440 - Misses 482 489 +7  Partials 113 113 

Flag	Coverage Δ
e2e-1.16.x	?
e2e-1.17.x	?
e2e-1.18.x	?
e2e-1.19.x	?
unittest-1.16.x	70.51% <76.19%> (-0.15%)	⬇️
unittest-1.17.x	70.51% <76.19%> (-0.15%)	⬇️
unittest-1.18.x	70.51% <76.19%> (-0.15%)	⬇️
unittest-1.19.x	70.51% <76.19%> (-0.25%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cover/instrument.go	20.83% <ø> (ø)
pkg/cover/store.go	81.45% <76.19%> (-2.58%)	⬇️

... and 1 file with indirect coverage changes