[enhancement]: stalwart-cli import messages - skip messages that already exist

[DSpeichert]

Which feature or improvement would you like to request?

I'd like to see this feature: stalwart-cli import messages should be able to skip importing messages that already exist.

Currently, stalwart-cli import messages will always import all messages, resulting in duplicates for multiple runs.

Is your feature request related to a problem?

Currently, aborted/crashed runs of stalwart-cli import messages cannot be resumed, nullifying the performance improvements of stalwart-cli import messages over tools like imapsync.

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