Keep timestamp of last syncRequest and update with xTime on each sync

The sync protocol with e.g. Qt 4 windows is broken if our app time is
older than the one of the last sync alarm event. Thus we keep a timestamp
in the syncRequest struct of the last sent sync request. If the timestamp
is newer than our xTime when sending the next request, we update the
xTime to ensure that we have a new timestamp again.

BUG: 333512
REVIEW: 117734