Restrict pgbench's zipfian parameter to ensure good performance.

Remove the code that supported zipfian distribution parameters less
than 1.0, as it had undocumented performance hazards, and it's not
clear that the case is useful enough to justify either fixing or
documenting those hazards.

Also, since the code path for parameter > 1.0 could perform badly
for values very close to 1.0, establish a minimum allowed value
of 1.001.  This solution seems superior to the previous vague
documentation warning about small values not performing well.

Fabien Coelho, per a gripe from Tomas Vondra

Discussion: https://postgr.es/m/b5e172e9-ad22-48a3-86a3-589afa20e8f7@2ndquadrant.com

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