config: replace fsnotify with reloading config file periodically

[djshow832]

What problem does this PR solve?

Issue Number: close #468

Problem Summary:
When the watched directory is removed and recreated immediately, sometimes fsnotify doesn't report an error and TiProxy is not aware of file change.

What is changed and how it works:
Simply reload the file periodically and set config if the file content changes.

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.

- Fix the bug that the config file may not be reloaded if the directory is removed and recreated immediately.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@6a3e4ce). Click here to learn what that means.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #475   +/-   ##
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  Coverage        ?   64.00%           
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  Files           ?       61           
  Lines           ?     5671           
  Branches        ?        0           
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  Hits            ?     3630           
  Misses          ?     1755           
  Partials        ?      286           

Flag	Coverage Δ
unit	64.00% <100.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-20 09:44:50.251278901 +0000 UTC m=+350378.998902012: ☑️ agreed by xhebox.