*: reduce memory allocation for plenty of idle connections

[djshow832]

What problem does this PR solve?

Issue Number: close #472

Problem Summary:
When there are many idle connections on a TiProxy instance, the CPU usage jitters every 2 minutes because of GC.

What is changed and how it works:

• Replace time.After to time.Ticker
• Replace sync.Map with map and sync.Lock
• Other optimizations like header, salt

Check List

Tests

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

Run and profile to see the frame graph and the overhead disappears.

alloc_objects:

inuse_objects:

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 GC CPU usage by 1% in the case of plenty of connections

[djshow832]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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Needs approval from an approver in each of these files:

• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-14 06:05:01.424832286 +0000 UTC m=+941626.991602177: ☑️ agreed by xhebox.
• 2024-02-20 09:58:05.041829415 +0000 UTC m=+351173.789452525: ✖️🔁 reset by djshow832.
• 2024-02-20 10:13:41.266714513 +0000 UTC m=+352110.014337625: ☑️ agreed by xhebox.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@7443ce6). Click here to learn what that means.

Files	Patch %	Lines
pkg/proxy/net/packetio.go	0.00%	0 Missing and 2 partials ⚠️
pkg/proxy/net/packetio_mysql.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #474   +/-   ##
=======================================
  Coverage        ?   64.25%           
=======================================
  Files           ?       61           
  Lines           ?     5698           
  Branches        ?        0           
=======================================
  Hits            ?     3661           
  Misses          ?     1752           
  Partials        ?      285           

Flag	Coverage Δ
unit	64.25% <84.00%> (?)

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