Use unsynchronized random number generator.

[cardbot]

The skiplist benchmark was using the global random number generator,
which is guarded by a mutex. This causes contention on multi-core
machines and skews the benchmark results in a material way. Instead,
use a separate random number generator per benchmark goroutine.
Update the README results.

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This change is 

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[manishrjain]

LGTM. Thanks for the change. Once you sign the CLA, I'll merge it.

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[cardbot]

@manishrjain, I signed the CLA, but it's not updating, even when I click the "Let us recheck it" link. So maybe I need to give it some time to refresh. Also, the CI tests failed in a way that looks unrelated to my change (flaky test?). I'm not sure how to re-run them.

[manishrjain]

Yeah, I can see that you signed. Not sure why CLA isn't updating.

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable