raft: benchmark results for ./benchmark put

I ran this PR against its main merge-base twice (on my 2021 Mac M1 pro),
and in both cases this PR was slightly faster, using the benchmark
invocation from [^1].

2819.6 vs 2808.4
2873.1 vs 2835

Full output below.

----

Script:

```
killall etcd
rm -rf default.etcd
scripts/build.sh
nohup ./bin/etcd  --quota-backend-bytes=4300000000 &
sleep 10
f=bench-$(git log -1 --pretty=%s | sed -E 's/[^A-Za-z0-9]+/_/g').txt
go run ./tools/benchmark txn-put --endpoints="http://127.0.0.1:2379" --clients=200 --conns=200 --key-space-size=4000000000 --key-size=128 --val-size=10240  --total=200000 --rate=40000 | tee "${f}"
```

PR:

```
Summary:
  Total:	70.9320 secs.
  Slowest:	0.3003 secs.
  Fastest:	0.0044 secs.
  Average:	0.0707 secs.
  Stddev:	0.0437 secs.
  Requests/sec:	2819.6030 (second run: 2873.0935)

Response time histogram:
  0.0044 [1]	|
  0.0340 [2877]	|
  0.0636 [119485]	|∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  0.0932 [17436]	|∎∎∎∎∎
  0.1228 [27364]	|∎∎∎∎∎∎∎∎∎
  0.1524 [20349]	|∎∎∎∎∎∎
  0.1820 [10214]	|∎∎∎
  0.2116 [1248]	|
  0.2412 [564]	|
  0.2707 [318]	|
  0.3003 [144]	|

Latency distribution:
  10% in 0.0368 secs.
  25% in 0.0381 secs.
  50% in 0.0416 secs.
  75% in 0.0998 secs.
  90% in 0.1375 secs.
  95% in 0.1571 secs.
  99% in 0.1850 secs.
  99.9% in 0.2650 secs.
```

main:

```
Summary:
  Total:	71.2152 secs.
  Slowest:	0.6926 secs.
  Fastest:	0.0040 secs.
  Average:	0.0710 secs.
  Stddev:	0.0461 secs.
  Requests/sec:	2808.3903 (second run: 2834.98)

Response time histogram:
  0.0040 [1]	|
  0.0728 [125816]	|∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  0.1417 [59127]	|∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  0.2105 [13476]	|∎∎∎∎
  0.2794 [1125]	|
  0.3483 [137]	|
  0.4171 [93]	|
  0.4860 [193]	|
  0.5549 [4]	|
  0.6237 [16]	|
  0.6926 [12]	|

Latency distribution:
  10% in 0.0367 secs.
  25% in 0.0379 secs.
  50% in 0.0417 secs.
  75% in 0.0993 secs.
  90% in 0.1367 secs.
  95% in 0.1567 secs.
  99% in 0.1957 secs.
  99.9% in 0.4361 secs.
```

[^1]: etcd-io#14394 (comment)

Signed-off-by: Tobias Grieger <tobias.b.grieger@gmail.com>