add single connection client for agent & vald

[kpango]

Signed-off-by: kpango kpango@vdaas.org

Description:

add single connection client for agent & vald

Related Issue:

How Has This Been Tested?:

Environment:

• Go Version: 1.16.4
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.12.3

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.

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[codecov]

Codecov Report

Merging #1254 (e8cea2e) into master (4e4367c) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1254      +/-   ##
==========================================
- Coverage   18.08%   17.89%   -0.19%     
==========================================
  Files         501      501              
  Lines       31303    31465     +162     
==========================================
- Hits         5660     5631      -29     
- Misses      25349    25535     +186     
- Partials      294      299       +5     

Impacted Files	Coverage Δ
internal/client/v1/client/agent/core/client.go	0.00% <0.00%> (ø)
internal/client/v1/client/vald/vald.go	0.00% <0.00%> (ø)
internal/net/grpc/client.go	0.00% <0.00%> (ø)
internal/db/storage/blob/s3/reader/reader.go	63.73% <0.00%> (-30.77%)	⬇️
internal/worker/queue.go	95.00% <0.00%> (-5.00%)	⬇️
internal/backoff/backoff.go	87.69% <0.00%> (+3.07%)	⬆️

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[hlts2]

Thank you.
LGTM

[kevindiu]

LGTM

[kpango]

@kevindiu @hlts2 Thanks for the review.
I will merge this PR after E2E tests have been completed excluding Redis&MySQL E2E.