[rate_ctr] always 'overflow' in next larger inetrval when interval ends

[rate_ctr] always 'overflow' in next larger inetrval when interval ends

If a second ends, we add the number of events in that just-ended second
to the number of events in the currently running minute.  The same happens
at the end of a minute: We add the number of events in that just-ended
minute into the number of events of the still-running hour, etc.

This gives a much more meaningful numbers and we don't end up with
"12 events per second, but 0 events per minute" kind of situations
anymore.