Correct comparison of System.nanoTime in SlidingTimeWindowReservoir

The currently used comparison between oldTick and Tick (tick > oldTick) is bound to break because System.nanoTime can easily overflow (more easily if multiplied with COLLISION_BUFFER). Therefore, one should compare tick - oldTick > 0, according to System.nanoTime documentation. This fixes an odd behaviour I have experienced where values wouldn't degrade after "leaving" the window.

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Thanks,
Volker