router, config: fix deadlock during closing the router

[djshow832]

What problem does this PR solve?

Issue Number: close #207

Problem Summary:

• Close the router may cause deadlock: router.Close() holds router.lock and waits for observer to finish; observer.observe() calls router.OnBackendChanged(), which waits for router.lock.
• If HealthCheck.Interval==0, even if HealthCheck.Enable==false, there is no interval for calling backendFetcher, which exhausts lots of resources.

What is changed and how it works:

• Remove the locks for StartScoreBasedRouter() and Close(). The lock intends to protect router.observer and router.cancelFunc but they are not very necessary.
• Add a HealthCheck.Check() when creating ScoreBasedRouter and PDFetcher.

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 (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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None