net: reduce the TLS buffer size to reduce memory usage

[djshow832]

What problem does this PR solve?

Issue Number: ref #1092

Problem Summary:
TLS also uses a buffer with the same size as basicReadWriter, which is unnecessary.

What is changed and how it works:
Use a smaller buffer size for TLS.

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

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

- reduce the TLS buffer size to reduce memory usage

[ti-chi-bot]

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[djshow832]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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[ti-chi-bot]

@djshow832: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-unit-test	5aa9673	link	true	/test pull-unit-test

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