pgbench: When using pipelining only do PQconsumeInput() when necessary.

pgbench: When using pipelining only do PQconsumeInput() when necessary.

Up to now we did a PQconsumeInput() for each pipelined query, asking the OS
for more input - which it often won't have, as all results might already have
been sent. That turns out to have a noticeable performance impact.

Alvaro Herrera reviewed the idea to add the PQisBusy() check, but not this
concrete patch.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210720180039.23rivhdft3l4mayn@alap3.anarazel.de
Backpatch: 14, where libpq/pgbench pipelining was introduced.