Function open_or_migrate_data_dir 

pub async fn open_or_migrate_data_dir(
    source_dir: &Path,
    destination_dir: &Path,
) -> Result<Option<StoreMigrationReceipt>, PortError>

Migrate once, reopen afterwards. Safe on every start: a destination that already holds a store is opened as it is, and the returned receipt says whether this call was the one that migrated it.