sqlreplay: do not write traffic files if meta exists

[djshow832]

What problem does this PR solve?

Issue Number: close #716

Problem Summary:
Writing meta may overwrite a system file with the same name, which can be used by attackers, so a precheck is needed before capturing.

What is changed and how it works:
Precheck the file meta before traffic capture. If the file exists, quit.

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.

- Do not start traffic capture if the file meta exists

[ti-chi-bot]

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[ti-chi-bot]

@djshow832: The following tests 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	55d6eb2	link	true	/test unit-test
pull-mysql-connector-test	55d6eb2	link	true	/test mysql-connector-test

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