ChunkedAssociativeLongArray re-use expired nodes

[storozhukBM]

This change gives ChunkedAssociativeLongArray ability to store chunks that are expired and removed from main data structure.
Then instead of allocating new Chunk immediately we are trying to poll one from this cache.
So if you have constant or slowly changing load ChunkedAssociativeLongArray will never throw away old chunks or allocate new ones which makes this data structure almost garbage free. Also chunks stored in cache as SoftReferences so this memory can actually be freed if nesesary.

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Numbers:
In ReservoirBenchmark: updates are 5.9x faster and ~370x less time spent in GC than SlidingTimeWindowReservoir.
In SlidingTimeWindowReservoirsBenchmark: updates are 3.2x faster, reads - 2.7x faster, 37x less time spent in GC. (here we see small difference because most time we spent in cleaning snapshots)

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Graphs:

Comparison of implementations under constant load [duration 1h]

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Comparison of implementations under changing load [duration 1h]

[storozhukBM]

Hi @arteam,
It would be nice if you have a chance to look at this PR before release.

[storozhukBM]

Here is great illustration of chunks reuse 😄 . link

[storozhukBM]

Hi @ryantenney, thank you for merge.
🎉

[ryantenney]

I didn't merge, but only because I didn't want to screw up the release @arteam was planning! Just waiting for him to review and merge!

[arteam]

What do you think about simplifying this method to:

while (true) {
  final SoftReference<Chunk> chunkRef = chunksCache.pollLast();
  if (chunkRef == null) {
    return new Chunk(defaultChunkSize);
  }
  final Chunk chunk = chunkRef.get();
  if (chunk != null) {
    chunk.cursor = 0;
    chunk.startIndex = 0;
    chunk.chunkSize = chunk.keys.length;
    return chunk;
  }
}

[arteam]

Please use multi-line comments with an asterix on every line. We are trying to follow the Google Java Style Guide. See section about comments.

[storozhukBM]

Hi @arteam,
Thank you for review. All comments fixed.