router: speed up health check by checking in parallel

[djshow832]

What problem does this PR solve?

Issue Number: close #216

Problem Summary:
In a large cluster, checking health serially is impractical.

What is changed and how it works:

• Define a wrapper for wait group and goroutine pool
• Check the health by the goroutine pool

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

- Speed up health check by checking in parallel

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.00000% with 3 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@d40bfdb). Click here to learn what that means.

Files	Patch %	Lines
pkg/manager/router/backend_observer.go	85.71%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #498   +/-   ##
=======================================
  Coverage        ?   64.83%           
=======================================
  Files           ?       61           
  Lines           ?     5846           
  Branches        ?        0           
=======================================
  Hits            ?     3790           
  Misses          ?     1762           
  Partials        ?      294           

Flag	Coverage Δ
unit	64.83% <94.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-08 04:47:25.095654868 +0000 UTC m=+851306.623195415: ☑️ agreed by xhebox.