@Override
      public void delete(K key) {
        callbackScheduler.deleteCallback(key);
      }
Returns a registry which allows registering arbitrary system-clock timer with K-typed key.
The user-defined function in the operator spec needs to implement {@link ScheduledFunction#onCallback(Object, long)}
for timer notifications.
@param <K> key type for the timer.
@return an instance of {@link Scheduler}
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImpl.java