core/timer: reset next_elapse_*time when timer is not waiting

core/timer: reset next_elapse_*time when timer is not waiting

When the unit that is triggered by a timer is started and running,
we transition to "running" state, and the timer will not elapse again
until the unit has finished running. In this state "systemctl list-timers"
would display the previously calculated next elapse time, which would
now of course be in the past, leading to nonsensical values.

Simply set the next elapse to infinity, which causes list-timers to
show n/a. We cannot specify when the next elapse will happen, possibly
never.

Fixes #4031.