Remove allocation in count_min_sketch::get_hashes

[cv4g]

Hi,

the countmin sketch is in use at ClickHouse and we noticed that there is a vector allocated on the hot path in count_min_sketch::get_hashes. To avoid it one could either inline it's body into the call sites or modify it to take a lambda that is invoked on each hash. Would you be interested in merging one of both approaches?

In a microbenchmark the change seems to yield an improvement for update between factor 1.1 and 1.4 and for estimate between factor 1.7 and 3.5:

   Group                    Time       CPU
  ─────────────────────  ────────  ────────
   UpdateUInt64           1.440x    1.440x
  ─────────────────────  ────────  ────────
   UpdateStringBytes      1.118x    1.119x
  ─────────────────────  ────────  ────────
   EstimateUInt64         3.548x    3.549x
  ─────────────────────  ────────  ────────
   EstimateStringBytes    1.764x    1.764x

The numbers were produced by a google-benchmark setup in #507.

Before:

Benchmark                                     Time             CPU   Iterations UserCounters...
-----------------------------------------------------------------------------------------------
BM_CountMinUpdateUInt64/1024              19585 ns        19606 ns        35834 bytes_per_second=398.467M/s items_per_second=52.2279M/s
BM_CountMinUpdateUInt64/4096              78486 ns        78515 ns         9017 bytes_per_second=398.011M/s items_per_second=52.1681M/s
BM_CountMinUpdateUInt64/32768            629480 ns       629521 ns         1103 bytes_per_second=397.127M/s items_per_second=52.0523M/s
BM_CountMinUpdateUInt64/65536           1252762 ns      1252815 ns          554 bytes_per_second=399.101M/s items_per_second=52.311M/s
BM_CountMinUpdateStringBytes/1024         34527 ns        34571 ns        20270 bytes_per_second=722.64M/s items_per_second=29.6201M/s
BM_CountMinUpdateStringBytes/4096        137133 ns       137181 ns         5092 bytes_per_second=739.661M/s items_per_second=29.8584M/s
BM_CountMinUpdateStringBytes/32768      1120784 ns      1120869 ns          632 bytes_per_second=749.194M/s items_per_second=29.2345M/s
BM_CountMinUpdateStringBytes/65536      2250362 ns      2250433 ns          310 bytes_per_second=759.885M/s items_per_second=29.1215M/s
BM_CountMinEstimateUInt64/1024            37893 ns        37893 ns        18530 bytes_per_second=206.172M/s items_per_second=27.0233M/s
BM_CountMinEstimateUInt64/4096           150433 ns       150432 ns         4647 bytes_per_second=207.734M/s items_per_second=27.2282M/s
BM_CountMinEstimateUInt64/32768         1216380 ns      1216377 ns          581 bytes_per_second=205.528M/s items_per_second=26.939M/s
BM_CountMinEstimateUInt64/65536         2419302 ns      2419296 ns          292 bytes_per_second=206.672M/s items_per_second=27.0889M/s
BM_CountMinEstimateStringBytes/1024       54027 ns        54026 ns        12960 bytes_per_second=462.411M/s items_per_second=18.9537M/s
BM_CountMinEstimateStringBytes/4096      215406 ns       215404 ns         3231 bytes_per_second=471.054M/s items_per_second=19.0154M/s
BM_CountMinEstimateStringBytes/32768    1762087 ns      1762084 ns          398 bytes_per_second=476.566M/s items_per_second=18.5962M/s
BM_CountMinEstimateStringBytes/65536    3541045 ns      3541037 ns          197 bytes_per_second=482.929M/s items_per_second=18.5076M/s

After:

Benchmark                                     Time             CPU   Iterations UserCounters...
-----------------------------------------------------------------------------------------------
BM_CountMinUpdateUInt64/1024              13339 ns        13362 ns        51549 bytes_per_second=584.679M/s items_per_second=76.635M/s
BM_CountMinUpdateUInt64/4096              55108 ns        55126 ns        12651 bytes_per_second=566.888M/s items_per_second=74.3031M/s
BM_CountMinUpdateUInt64/32768            436977 ns       436969 ns         1597 bytes_per_second=572.123M/s items_per_second=74.9893M/s
BM_CountMinUpdateUInt64/65536            877659 ns       877601 ns          808 bytes_per_second=569.735M/s items_per_second=74.6763M/s
BM_CountMinUpdateStringBytes/1024         30439 ns        30472 ns        23111 bytes_per_second=819.845M/s items_per_second=33.6044M/s
BM_CountMinUpdateStringBytes/4096        123387 ns       123418 ns         5643 bytes_per_second=822.14M/s items_per_second=33.188M/s
BM_CountMinUpdateStringBytes/32768      1001801 ns      1001729 ns          711 bytes_per_second=838.299M/s items_per_second=32.7115M/s
BM_CountMinUpdateStringBytes/65536      2029662 ns      2028176 ns          351 bytes_per_second=843.157M/s items_per_second=32.3128M/s
BM_CountMinEstimateUInt64/1024            10599 ns        10598 ns        64017 bytes_per_second=737.185M/s items_per_second=96.6243M/s
BM_CountMinEstimateUInt64/4096            43193 ns        43185 ns        16224 bytes_per_second=723.625M/s items_per_second=94.8469M/s
BM_CountMinEstimateUInt64/32768          339303 ns       339262 ns         2040 bytes_per_second=736.894M/s items_per_second=96.5861M/s
BM_CountMinEstimateUInt64/65536          681292 ns       681216 ns         1031 bytes_per_second=733.981M/s items_per_second=96.2044M/s
BM_CountMinEstimateStringBytes/1024       30358 ns        30355 ns        23095 bytes_per_second=823.022M/s items_per_second=33.7346M/s
BM_CountMinEstimateStringBytes/4096      122105 ns       122090 ns         5731 bytes_per_second=831.082M/s items_per_second=33.5489M/s
BM_CountMinEstimateStringBytes/32768    1003445 ns      1003328 ns          691 bytes_per_second=836.963M/s items_per_second=32.6593M/s
BM_CountMinEstimateStringBytes/65536    2016679 ns      2016441 ns          345 bytes_per_second=848.064M/s items_per_second=32.5008M/s