router, backend: refine some logs

[djshow832]

What problem does this PR solve?

Issue Number: close #218

Problem Summary:
Some problems of logs:

• some are redundant
• not easy to search
• lack of information

What is changed and how it works:

• Uniform keywords like update backend, client_addr, backend_addr, connID for easier search
• Print the scores of source and goal backend before redirection
• Change some logs to debug level to reduce logs

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

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None