core: Make sure a stamp file exists for all Persistent=true timers

core: Make sure a stamp file exists for all Persistent=true timers

If a persistent timer has no stamp file yet, it behaves just like a normal
timer until it runs for the first time. If the system is always shut down
while the timer is supposed to run, a stamp file is never created and
Peristent=true has no effect.

This patch fixes this by creating a stamp file with the current time
when the timer is first started.