server: graceful shutdown notice in HTTP

[xhebox]

What problem does this PR solve?

Issue Number: ref #245

Problem Summary: Return 504 when closing. Also trying to fix post merge CI & provide cross-platform cache.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

[xhe@M1PRO tiproxy]$ ./bin/tiproxyctl health
{"config_checksum":4122079757}
[xhe@M1PRO tiproxy]$ ./bin/tiproxyctl health
502 Bad Gateway: {"config_checksum":4122079757}

• 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.

Return 504 when closing server.