net: pool bufio reader and writer to reduce memory allocation (#776)

[Samriddha9619]

What problem does this PR solve?

Issue Number: close #776

Problem Summary:
In short-lived connection workloads, buffered IO is created frequently, leading to heavy memory allocation and GC pressure.

What is changed and how it works:

• Implemented sync.Pool for bufio.Reader and bufio.Writer in basicReadWriter.
• Added a pooled flag to safely manage the buffer lifecycle.
• Implemented freeBasicReadWriter to recursively unwrap connection layers (TLS, compression) and return standard-sized buffers to the pool on connection close to prevent memory leaks.

Check List

Tests - [x] 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

Please refer to Release Notes Language Style Guide to write a quality release note.

Optimize memory allocation by pooling buffered IO for client connections

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[ti-chi-bot]

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[ti-chi-bot]

Welcome @Samriddha9619! It looks like this is your first PR to pingcap/tiproxy 🎉

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