fix(manager): use mutex instead of atomic batchInProcess

[danwt]

PR Standards

Opening a pull request should be able to meet the following requirements

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Close #675

<-- Briefly describe the content of this pull request -->

For Author:

• Targeted PR against correct branch
• included the correct type prefix in the PR title
• Linked to Github issue with discussion and accepted design
• Targets only one github issue
• Wrote unit and integration tests
• All CI checks have passed
• Added relevant godoc comments

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For Reviewer:

• confirmed the correct type prefix in the PR title
• Reviewers assigned
• confirmed all author checklist items have been addressed

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After reviewer approval:

• In case targets main branch, PR should be squashed and merged.
• In case PR targets a release branch, PR should be rebased.