unlock of free user KDBXes unnecessarily include the 2nd read operation

[luckyrat]

We only need this 2nd read for subscribed users that might be working on their Vault while a new one is being downloaded and merged in the background (or in the rare case when a free user first attaches to an existing remote account). So we could defer the read/unlock effort until really needed if we know we have no current User.

Low priority since we will first investigate if we can make changes to the sync operations so that they can be really done as a background operation while the app is not even open and thus remove this 2nd read operation for all users at the same time.