log_backup: fix panic when encountered error during resuming (#17021)

[ti-chi-bot]

This is an automated cherry-pick of #17021

What is changed and how it works?

Issue Number: Close #17020

What's Changed:

Spawn thread from the thread pool directly. (Instead of the thread local runtime handle.)

Related changes

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

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

Release note

Fixed a bug that might cause TiKV panic when resuming a PiTR task and the connection to PD isn't stable enought.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Leavrth
• YuJuncen

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[glorv]

/test

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[glorv]

/test

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[glorv]

/merge

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

Commit hash: ed27edb