router: refresh backends when the backends are empty

[djshow832]

What problem does this PR solve?

Issue Number: close #168

Problem Summary:
When 2 TiDB instances are rolling restart, the TiProxy may think both of them are down at one moment. In this case, the TiProxy should refresh the backends immediately.

What is changed and how it works:

• Do the health check again when there is no backends to return
• Change the DialTimeout to 2 seconds

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