core/timer: Always use inactive_exit_timestamp if it is set

core/timer: Always use inactive_exit_timestamp if it is set

If we're doing a daemon-reload, we'll be going from TIMER_DEAD => TIMER_WAITING,
so we won't use inactive_exit_timestamp because TIMER_DEAD != UNIT_ACTIVE, even
though inactive_exit_timestamp is serialized/deserialized and will be valid after
the daemon-reload.

This issue can lead to timers never firing as we'll always calculate the next
elapse based on the current realtime on daemon-reload, so if daemon-reload happens
often enough, the elapse interval will be moved into the future every time, which
means the timer will never trigger.

To fix the issue, let's always use inactive_exit_timestamp if it is set, and only
fall back to the current realtime if it is not set.