Add more effective Close function for internal/core

[kpango]

Signed-off-by: kpango i.can.feel.gravity@gmail.com

Description:

Related Issue:

How Has This Been Tested?:

Environment:

• Go Version: 1.15.2
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.12.1

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.

[vdaas-ci]

[CHATOPS:HELP] ChatOps commands.

• 🙆‍♀️ /approve - approve
• 💌 /changelog - add changelog comment
• 🍱 /format - format codes and add licenses
• ✅ /gen-test - generate test codes
• 🏷️ /label - add labels
• ⏪ /rebase - rebase master

[codecov]

Codecov Report

Merging #764 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #764      +/-   ##
==========================================
- Coverage   16.77%   16.73%   -0.04%     
==========================================
  Files         421      420       -1     
  Lines       19530    19476      -54     
==========================================
- Hits         3276     3260      -16     
+ Misses      16025    15995      -30     
+ Partials      229      221       -8     

Impacted Files	Coverage Δ
internal/net/net.go
internal/worker/worker.go	83.33% <0.00%> (+1.04%)	⬆️
internal/errgroup/group.go	93.93% <0.00%> (+1.51%)	⬆️
internal/db/storage/blob/s3/reader/reader.go	100.00% <0.00%> (+32.55%)	⬆️

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