[crl-release-21.1] *: Skip first iteration of read sample

[itsbilal]

Backport of #1098.

---

Currently, we always sample on the first read in a newly-opened
iterator, for determining read compactions. This is because the
zero value of bytesUntilReadSample
is always less than the bytes read. In cases where iterators are
short-lived, such as in ycsb workloads, this makes a greater proportion
of reads sampled than we'd like them to be, and it also makes any
changes to the sampling rate less effective.

This change updates Iterator to detect and skip the first time
it tries to sample a read on a newly-opened iterator.

Also makes a change to use ints instead of uints when
copying over options, to allow for disabling read sampling
by specifying a negative value for ReadSamplingMultiplier
if necessary.

When this change was tested with cockroachdb master, this
performance improvement was had on roachtests compared to
a clean master:

name            old ops/sec  new ops/sec  delta
ycsb/A/nodes=3   17.6k ± 9%   20.1k ± 5%  +14.19%  (p=0.008 n=5+5)
ycsb/B/nodes=3   33.7k ± 5%   35.6k ± 5%   +5.61%  (p=0.032 n=5+5)
ycsb/C/nodes=3   45.2k ± 3%   44.6k ± 3%     ~     (p=0.421 n=5+5)

name            old p50      new p50      delta
ycsb/A/nodes=3    3.56 ± 7%    3.14 ± 5%  -11.80%  (p=0.024 n=5+5)
ycsb/B/nodes=3    3.00 ± 0%    2.84 ± 8%     ~     (p=0.095 n=4+5)
ycsb/C/nodes=3    2.72 ± 4%    2.72 ± 4%     ~     (p=1.000 n=5+5)

name            old p95      new p95      delta
ycsb/A/nodes=3    17.1 ±11%    14.8 ± 4%  -13.35%  (p=0.024 n=5+5)
ycsb/B/nodes=3    13.6 ± 0%    13.0 ± 7%     ~     (p=0.095 n=4+5)
ycsb/C/nodes=3    8.90 ± 0%    8.90 ± 0%     ~     (all equal)

name            old p99      new p99      delta
ycsb/A/nodes=3    32.9 ±27%    27.5 ±18%  -16.52%  (p=0.048 n=5+5)
ycsb/B/nodes=3    25.8 ± 7%    24.3 ±16%     ~     (p=0.302 n=5+5)
ycsb/C/nodes=3    13.6 ± 4%    14.0 ± 3%     ~     (p=0.365 n=5+5)

[cockroach-teamcity]

This change is 

[jbowens]

oh I see, you already put this up :)

Reviewable status: 0 of 3 files reviewed, all discussions resolved (waiting on @sumeerbhola)

[itsbilal]

Hah yep, TFTR!