timesyncd: write structured log messages whenever we bump the clock based on disk...

timesyncd: write structured log messages whenever we bump the clock based on disk timestamp

It's useful being able to easily detect if a disk-based clock bump was
done, let's make it a structure message, the same way as acquiring an
NTP fix already is.

Also, set the clock to 1 µs further than the timestamp from the disk,
after all we know that that timestamp was current when it was written,
hence it can't be the right one right now anymore.