Remove now-unnecessary thread pointer arguments in pgbench.
Not required after nuking the zipfian thread-local cache.
Also add a comment about hazardous pointer punning in threadRun(),
and avoid using "thread" to refer to the threads array as a whole.
Fabien Coelho and Tom Lane, per suggestion from Alvaro Herrera
Discussion: https://postgr.es/m/alpine.DEB.2.21.
1904032126060.7997@lancre