proxy: request quickack before frontend reads

[baiyuqing]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:

In the serverless gateway path, the frontend socket can occasionally delay pure ACKs long enough for clients to hit the Linux ~200ms retransmission window. That shows up as sporadic max-latency spikes even when TiDB itself is stable.

What is changed and how it works:

• Add a frontend-only PacketIO option to request TCP_QUICKACK
• Enable that option for client-facing connections by default
• Request quickack before each frontend ReadPacket() so the socket re-enters quickack mode after the kernel leaves it
• Keep the behavior Linux-only; non-Linux builds are a no-op
• Leave backend connection behavior unchanged

This keeps the change narrowly scoped to the client -> TiProxy side where delayed ACKs were suspected to amplify tail latency.

Check List

Tests

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

Test details:

• go test ./pkg/proxy/net ./pkg/proxy/client ./pkg/proxy
• Downstream verification in aws-shared-provider: GOSUMDB=off go test ./pkg/gateway/... ./cmd/tidb-gateway/...

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Reduce frontend tail latency spikes by requesting TCP_QUICKACK before reading client packets.

[ti-chi-bot]

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