Optimize Snapshot class constructor

In some of our workloads (where we need to gather samza metrics five times per minute) `SlidingTimeWindowReservoir.getSnapshot()` method takes up to 10% of processor time.

Almost all of `getSnapshot` time is taken by Collections.sort method. So, the complexity of Snapshot constructor is O(NlogN) + iteration through passed values.

This ticket asks to improve the performance of Snapshot constructor but keep the performance of all other methods at least on the same level.