config: revert to watch config file and fix the concurrency issue

[djshow832]

What problem does this PR solve?

Issue Number: close #444

Problem Summary:
#445 has another bug: the accuracy of file modify time is 1ms. The file may be updated twice in 1ms and TiProxy may fail to load the latest one.

What is changed and how it works:

• Revert to the watch config file way
• Fix the concurrency issue in this way: if there's a watch error, in the next tick, if adding the watch file succeeds, reload it.
• Make the tick interval configurable and set it to 1s by default

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.

None

[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:

• 2024-01-04 02:44:45.367202503 +0000 UTC m=+2311376.404429430: ☑️ agreed by xhebox.