fix(compactor): Fix an unexpected termination of shared compactor

[Li0k]

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What's changed and what's your intention?

If the shutdown_tx returned by the function is not retained, then shutdown_rx will receive an exit message and the shared compactor loop will be terminated.

This pr maintains the lifetime of the start_shared_compactor return value to avoid this problem.

This PR is just a hot fix, and does not prevent serverless compactor v2.0.x from being tested.
If necessary, a separate PR will be launched to address the compactor graceful shutdown and cancel task issues.

Checklist

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Release note

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