Compare System.nanoTime by (tick - oldTick > 0) instead of (tick > oldTick) in SlidingTimeWindowReservoir

Initialized lastTick with (System.nanoTime * COLLISION_BUFFER) and changed
comparison as indicated above. Otherwise, newTick will always be (oldTick+1)
due to (System.nanoTime * COLLISION_BUFFER) easily resulting in a
big, negative, long.
Merge pull request #623 from volker/master

Correct comparison of System.nanoTime in SlidingTimeWindowReservoir