manager: use max of: compile epoch, epoch file, timesyncd file

manager: use max of: compile epoch, epoch file, timesyncd file

Previously systemd would not use /var/lib/systemd/timesync/clock. This means
that even if /var/ is mounted when systemd is started and the file is
available, we would potentially make one time jump and than another time jump.
From a user's POV, this doesn't seem useful at all.

Also, we would always let /usr/lib/clock-epoch take priority over the built-in
epoch. But there is no guarantee that this file is actually fresh. In
particular, a user may touch /usr/lib/clock-epoch to work around a broken clock
during installation (as recommended in [1]), and then this file will grow stale
over time.

So just load the three timestamps and use the highest one as the epoch.

[1] https://discussion.fedoraproject.org/t/f38-to-f39-40-dnf-system-upgrade-can-fail-on-raspberry-pi/92403