server, sessionctx: improved mysql compatibility with support for init_connect (#23713)

[ti-srebot]

cherry-pick #23713 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26031

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---

What problem does this PR solve?

Issue Number: close #18894

Problem Summary:

MySQL supports the ability to execute a set of statements on first connection. This PR adds the same feature to TiDB.

It can be useful for some applications, since SQL modes can be set, user variables, etc. It is also used extensively in the mysql-test testsuite.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Integration test: server integration tests.

Side effects

• Performance regression
  • Regrettably checking if initconnect returned an error is in the query-serving path. This was required to return the client an error.

Release note

• TiDB now supports the mysql system variable init_connect and associated functionality.

[ti-srebot]

/run-all-tests

[ti-srebot]

@morgo please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

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[ti-chi-bot]

[REVIEW NOTIFICATION]

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[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 22da90a

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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