*: tls refactor

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: ref #136

Problem Summary: proxy.NewSQLServer still relies heavily on certManager. This is a preparation for the later PR.

CertManager is largely simplified by moving reloading functions into certInfo, which is moved to /lib package. I expect that:

1. proxy.NewSQLServer could workaround certManager by interfaces hack
2. per-namespace TLS will replace certManager eventually

Some minor but function-neutral changes are done to the certInfo. And AutoTLS is replaced by CreateTempTLS, i.e. auto tls rotation is now done in memory completely.

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