Always keep the previous sync key

If a sync command fails, we need to be able to retry.
I assume the intention was to never loose uploaded data,
but if the upload fails the client must resend the data,
so we should always remember the previous sync key.

With this change, it is however possible now to send the same sync-key
several times and append more data, which mustn't be possible.
I suppose we could simply assume that if we already know the sync-key, we
already have the data, and therefore ignore the resent data.
But we also need to make sure that we really do have the data once we
store the new sync key.