Reuse blockkey, speeds up large Update and Replace calls
Reuse blockkey, speeds up large Update and Replace calls

benchmark                         old ns/op      new ns/op      delta
BenchmarkReplaceAll-8             2866418930     2880834572     +0.50%
BenchmarkUpdateOneChanged-8       226635         236596         +4.40%
BenchmarkUpdateOneUnchanged-8     229090         227326         -0.77%
BenchmarkNeedHalf-8               104483393      105151538      +0.64%
BenchmarkHave-8                   29288220       28827492       -1.57%
BenchmarkGlobal-8                 159269126      150768724      -5.34%
BenchmarkNeedHalfTruncated-8      108235000      104434216      -3.51%
BenchmarkHaveTruncated-8          28945489       27860093       -3.75%
BenchmarkGlobalTruncated-8        149355833      149972888      +0.41%

benchmark                         old allocs     new allocs     delta
BenchmarkReplaceAll-8             1054944        555451         -47.35%
BenchmarkUpdateOneChanged-8       1135           1135           +0.00%
BenchmarkUpdateOneUnchanged-8     1135           1135           +0.00%
BenchmarkNeedHalf-8               374777         374779         +0.00%
BenchmarkHave-8                   151995         151996         +0.00%
BenchmarkGlobal-8                 530063         530066         +0.00%
BenchmarkNeedHalfTruncated-8      374699         374702         +0.00%
BenchmarkHaveTruncated-8          151834         151834         +0.00%
BenchmarkGlobalTruncated-8        530021         530049         +0.01%

benchmark                         old bytes      new bytes      delta
BenchmarkReplaceAll-8             5074297112     5018351912     -1.10%
BenchmarkUpdateOneChanged-8       135097         135085         -0.01%
BenchmarkUpdateOneUnchanged-8     134976         134976         +0.00%
BenchmarkNeedHalf-8               44759436       44769400       +0.02%
BenchmarkHave-8                   11911138       11930612       +0.16%
BenchmarkGlobal-8                 81609867       81523668       -0.11%
BenchmarkNeedHalfTruncated-8      46588024       46692342       +0.22%
BenchmarkHaveTruncated-8          11348354       11348357       +0.00%
BenchmarkGlobalTruncated-8        79485168       81843956       +2.97%

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