Allow to persist QoS0 messages to disk in case of upgrade_qos

[mths1]

Proposed Changes

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• Bugfix (non-breaking change which fixes issue [Bug]: Outgoing QoS upgrades not affecting storage of offline messages #2220 )
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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[ioolkos]

I think we can merge this. Ideally, we'd have a test for upgrade_qos behaviour (I checked, we never had one)