gcs: Implement version 2 filters.

[davecgh]

This requires #1851, and #1854.

This implements new version 2 filters which have 4 changes as compared to version 1 filters:

• Support for independently specifying the false positive rate and Golomb coding bin size which allows minimizing the filter size
• A faster (incompatible with version 1) reduction function
• A more compact serialization for the number of members in the set
• Deduplication of all hash collisions prior to reducing and serializing the deltas

In addition, it adds a full set of tests and updates the benchmarks to use the new version 2 filters.

The primary motivating factor for these changes is the ability to minimize the size of the filters, however, the following is a before and after comparison of version 1 and 2 filters in terms of performance and allocations.

It is interesting to note the results for attempting to match a single item is not very representative due to the fact the actual hash value itself dominates to the point it can significantly vary due to the very low ns timings involved. Those differences average out when matching multiple items, which is the much more realistic scenario, and the performance increase is in line with the expected values. It is also worth nothing that filter construction now takes a bit longer due to the additional deduplication step. While the performance numbers for filter construction are about 25% larger in relative terms, it is only a few ms difference in practice and therefore is an acceptable trade off for the size savings provided.

benchmark                      old ns/op    new ns/op    delta
-----------------------------------------------------------------
BenchmarkFilterBuild50000      16194920     20279043     +25.22%
BenchmarkFilterBuild100000     32609930     41629998     +27.66%
BenchmarkFilterMatch           620          593          -4.35%
BenchmarkFilterMatchAny        2687         2302         -14.33%

benchmark                      old allocs   new allocs   delta
-----------------------------------------------------------------
BenchmarkFilterBuild50000      6            17           +183.33%
BenchmarkFilterBuild100000     6            18           +200.00%
BenchmarkFilterMatch           0            0            +0.00%
BenchmarkFilterMatchAny        0            0            +0.00%

benchmark                      old bytes    new bytes    delta
-----------------------------------------------------------------
BenchmarkFilterBuild50000      688366       2074653      +201.39%
BenchmarkFilterBuild100000     1360064      4132627      +203.86%
BenchmarkFilterMatch           0            0            +0.00%
BenchmarkFilterMatchAny        0            0            +0.00%

[davecgh]

This has been updated to support deduplication of filter data for the version 2 filters. The PR description and all benchmarks have also been updated accordingly.

For a concrete example of the difference the deduplication can make, consider the following values for block 1096:

v1 filter bytes: 2940
v2 filter bytes before update: 2819
v2 filter bytes after update: 69

[dnldd]

Looks good.