sql: move benchmark tests to a new bench package

[knz]

As suggested by @jordanlewis here and here.

[cockroach-teamcity]

This change is 

[jordanlewis]

Please post a quick PSA reminding people that to use the SQL benchmarks you'll now need to speicfy the pkg/bench package instead of pkg/sql/bench- normally I wouldn't care but the command line invocations for these sql benchmarks are very magic and ./scripts/bench makes it hard to see what you did wrong.

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Reviewed 14 of 14 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[knz]

Thank you!