puller: fix dead lock

[lidezhu]

What problem does this PR solve?

Issue Number: close #1081

What is changed and how it works?

1. We have limited go routines to schedule range request, and they may be blocked on lock ranges.
2. These ranges may be locked by other stale regions, and the ranges can only be unlocked by doHandleError.
3. doHandleError may blocked on sending rangeTask, because all the go routines are blocked on lock ranges and cannot handle the new rangeTask.

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

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If you don't think this PR needs a release note then fill it with `None`.

[lidezhu]

/run-check-issue-triage-complete

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: asddongmen

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-03-12 01:33:59.843224654 +0000 UTC m=+319595.448753583: ☑️ agreed by asddongmen.