prepare_snap: establish connection to all stores before pausing admin (#51449)

[ti-chi-bot]

This is an automated cherry-pick of #51449

What problem does this PR solve?

Issue Number: close #51448

Problem Summary:
Now if we cannot connect to a TiKV, it will block at creating connection.
But, in this scenario, some of TiKVs' admin commands are paused. That will impact the TP workload more.

What changed and how does it work?

After this PR, we won't pause the admin commands before connections to all TiKV nodes established.

Check List

Tests

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

  • I checked and no code files have been changed.

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	8c0e1ab	link	true	/test unit-test

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[YuJuncen]

should be picked in #53027