[improve][broker] make split namespaces bundle async

[leizhiyuan]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #15043

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Master Issue: #

Motivation

make split namespaces bundle async

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Modifications

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Verifying this change

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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• The schema: (yes / no / don't know)
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• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

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[github-actions]

@leizhiyuan:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@leizhiyuan:Thanks for providing doc info!

[gaozhangmin]

LGTM

[github-actions]

The pr had no activity for 30 days, mark with Stale label.

[leizhiyuan]

see #15043

[leizhiyuan]

#16411 has been merged ..