infoschema: maintain the snapshotTS in the infocache (#25322)

[ti-srebot]

cherry-pick #25322 to release-5.1
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git pr https://github.com/pingcap/tidb/pull/25376

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---

Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

Issue Number: #25303

Problem Summary:
Infoschema cache is introduced by #24285, which mainly used for stale read and history read. It let executor can get the corresponding history schema. But even if we have the cache, every request still needs to go to TiKV to obtain the schema version to confirm the corresponding schema. which leads one TiKV to have many KV_Get, This may let one TiKV who has the meta region's leader consume too much CPU and become a bottleneck. And intuitively, the request is unbalanced:

What is changed and how it works?

• Cache the relative ts in the infocache. each schema has only corresponded sanpshotTS, which is the oldest ts we have used with the schema.
• Make the request can get the Infoschema directly with the specified timestamp.

Check List

Tests

• Unit test

Release note

• None

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[nolouch]

PTAL @xhebox @Yisaer

[ti-chi-bot]

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[Yisaer]

/merge

[ti-chi-bot]

@Yisaer: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

In response to this:

/merge

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[nolouch]

/merge

[ti-chi-bot]

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

Commit hash: 2dcbe9e

[xhebox]

/merge

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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