Reuse buffer for hash computation in the engine.

[cyriltovena]

Signed-off-by: Cyril Tovena cyril.tovena@gmail.com

❯ benchcmp old.txt new.txt
benchmark                         old ns/op      new ns/op      delta
BenchmarkRangeQuery100000-16      3282273        2946683        -10.22%
BenchmarkRangeQuery200000-16      15338653       15573954       +1.53%
BenchmarkRangeQuery500000-16      2025848024     1976072536     -2.46%
BenchmarkRangeQuery1000000-16     3964696705     3868484541     -2.43%

benchmark                         old allocs     new allocs     delta
BenchmarkRangeQuery100000-16      5244           5296           +0.99%
BenchmarkRangeQuery200000-16      5932           6025           +1.57%
BenchmarkRangeQuery500000-16      138655         138727         +0.05%
BenchmarkRangeQuery1000000-16     271995         272065         +0.03%

benchmark                         old bytes     new bytes     delta
BenchmarkRangeQuery100000-16      719383        720496        +0.15%
BenchmarkRangeQuery200000-16      1404153       1446713       +3.03%
BenchmarkRangeQuery500000-16      135373656     135393336     +0.01%
BenchmarkRangeQuery1000000-16     270947216     270966704     +0.01%

Allocations are a bit more but this is because we need to improve a bit our benchmark to use more labels, currently use only foo/bar...