core: don't drop timer expired but not yet processed when system date is changed

core: don't drop timer expired but not yet processed when system date is changed

There is difference between time set by the user and real elapsed time because of accuracy feature.
If you change the system date(or time) between these times, the timer drops.

You can easily reproduce it with the following command.
-----------------------------------------------------------
$ systemd-run --on-active=3s ls; sleep 3; date -s "`date`"
-----------------------------------------------------------

In the following command, the problem is rarely reproduced. But it exists.
---------------------------------------------------------------------------------------------
$ systemd-run --on-active=3s --timer-property=AccuracySec=1us ls ; sleep 1; date -s "`date`"
---------------------------------------------------------------------------------------------

Note : Global AccuracySec value.
----------------------------------------------------------------------
$ cat /etc/systemd/system.conf
DefaultTimerAccuracySec=1min
----------------------------------------------------------------------