Handle time period elapses in backend cache

[philipgough]

When we detect a time period has elapsed, we might want to push the counters onto the queue so that they can be flushed at a later time.

Only the lowest counter that expires should be pushed to the queue and have the counter reset. We will need to keep the periods that have not yet expired.

This involves watching the clock and being notified of elapsed periods, for example, when we get :59 we elapse a minute, push the minute counters to the queue and reset the counters for the minute period. The same would occur for an hour at :00