session, statistics: fix "data too long for column 'xxx_bound'" when analyze tables with very long strings (#28800)

[ti-srebot]

cherry-pick #28800 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/29072

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/29072:release-5.1-de9b5cbb051f

---

What problem does this PR solve?

Issue Number: close #28793

Several columns in stats-related system tables are not long enough to hold long strings.

What is changed and how it works?

Change these columns from blob to longblob.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix "Data too long for column 'xxx_bound'" error when analyzing a table with strings longer than 65535 bytes.

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@time-and-fate you're already a collaborator in bot's repo.

[wjhuang2016]

Please resolve the conflict.

[time-and-fate]

Please resolve the conflict.

We are still not sure if this PR needs to be merged because (1) this bug has a simple workaround (2) cherry-picking this will be very complicated since we need to handle the bootstrap version on 5.1, 5.2, 5.3, and master carefully.