Add query cancellation capabilities in pgbench init phase

Add query cancellation capabilities in pgbench init phase

This can be useful to stop data generation happening on the server for
long-running queries caused by large scale factors.  This cannot happen
by default as data is generated on the client, but it is possible to
control the initialization steps of pgbench to do that.

Reported-by: Fujii Masao
Author: Fabien Coelho
Discussion: https://postgr.es/m/alpine.DEB.2.21.1910311939430.27369@lancre
Discussion: https://postgr.es/m/CAHGQGwHWEyTXxZh46qgFY8a2bDF_EYeUdp3+_Hy=qLZSzwVPKg@mail.gmail.com