Server: Introduce proxy state machine (#9856)

[ti-chi-bot]

This is an automated cherry-pick of #9856

What problem does this PR solve?

Issue Number: close #9857 ref #9855

Problem Summary:

This is a trivial refinement of code to make proxy codes not so tangled with in Server::main.

This endeavor is to start TiFlash storage layer alone without Proxy and KVStore module, and make further cp easy.

We'd like to cp this PR into release-8.5 in order to keep compatible with cse TiFlash.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

@CalvinNeo This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[JaySon-Huang]

Replaced by #9863