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.
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$ systemd-run --on-active=3s ls; sleep 3; date -s "`date`"
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In the following command, the problem is rarely reproduced. But it exists.
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$ systemd-run --on-active=3s --timer-property=AccuracySec=1us ls ; sleep 1; date -s "`date`"
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Note : Global AccuracySec value.
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$ cat /etc/systemd/system.conf
DefaultTimerAccuracySec=1min
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Origin source: https://github.com/systemd/systemd/commit/fee04d7f3ab810e99b97535ca5fda2f9517acda9

Change-Id: Ibaaa89bec9c55f43336388ee85f07b89c1a50ea4
Signed-off-by: Hyotaek Shim <hyotaek.shim@samsung.com>