backend, cli: fix vulnerability issues scanned by third-party tools

[djshow832]

What problem does this PR solve?

Issue Number: close #620

Problem Summary:
Third-party tools treat math/rand and unsafe as insecure packages.

What is changed and how it works:

• Replace unsafe
• Remove math/rand

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.

None

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #623   +/-   ##
=======================================
  Coverage        ?   68.74%           
=======================================
  Files           ?       85           
  Lines           ?     7955           
  Branches        ?        0           
=======================================
  Hits            ?     5469           
  Misses          ?     2097           
  Partials        ?      389           

Flag	Coverage Δ
unit	68.74% <100.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-05 07:26:14.452318104 +0000 UTC m=+250504.319417193: ☑️ agreed by xhebox.