pgsql: pgbench: When using pipelining only do PQconsumeInput() when nec

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(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20210720180039.23rivhdft3l4mayn@alap3.anarazel.de
Backpatch: 14, where libpq/pgbench pipelining was introduced.

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fa604e0dd07a39ba34f93d06ded8243280dffdeb

Modified Files
--------------
src/bin/pgbench/pgbench.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)