[bug fix] Fix global cache in internal/cache/gache package

[kevindiu]

Description:

This PR fixed the global cache bug in internal package.
Since the defaultOps is a global instance and created only once, the instance will be shared among all the instance using gache or cache package.

This bug was discovered in #501.
This PR does not include the test code.

Related Issue:

How Has This Been Tested?:

Environment:

• Go Version: 1.14.3
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.11.5

Types of changes:

• Bug fix [type/bug]
• New feature [type/feature]
• Add tests [type/test]
• Security related changes [type/security]
• Add documents [type/documentation]
• Refactoring [type/refactoring]
• Update dependencies [type/dependency]
• Update benchmarks and performances [type/bench]
• Update CI [type/ci]

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

Checklist:

• I have read the CONTRIBUTING document.
• I have checked open Pull Requests for the similar feature or fixes?
• I have added tests and benchmarks to cover my changes.
• I have ensured all new and existing tests passed.
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly.

[pull-assistant]

Best reviewed: commit by commit

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Optimal code review plan

     fix global cache

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[vdaas-ci]

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[codecov]

Codecov Report

Merging #560 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master    #560      +/-   ##
=========================================
- Coverage    9.79%   9.78%   -0.02%     
=========================================
  Files         402     402              
  Lines       20859   20862       +3     
=========================================
- Hits         2044    2042       -2     
- Misses      18558   18564       +6     
+ Partials      257     256       -1     

Impacted Files	Coverage Δ
internal/cache/gache/gache.go	0.00% <0.00%> (ø)
internal/cache/gache/option.go	0.00% <0.00%> (-6.67%)	⬇️
internal/errgroup/group.go	93.42% <0.00%> (-1.32%)	⬇️

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[kpango]

LGTM