tls: fix server side verification

[xhebox]

What problem does this PR solve?

Issue Number: close #220

Problem Summary: Due to wrong ClientAuth, it is not working as expected. With VerifyClientCertIfGiven, the verify function from golang will be invoked using empty CA list, resulting into some errors.

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

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

None